Road gradient estimation arbitration

ABSTRACT

A vehicle and vehicle system are provided with a controller that is configured to generate output indicative of a road gradient based on at least one of a first estimation, a second estimation and a third estimation. The road gradient is based on the first estimation when a vehicle speed is less than a speed threshold and an input indicative of a longitudinal acceleration is available. The road gradient is based on the second estimation when the vehicle speed is greater than the speed threshold and the longitudinal acceleration is available. The road gradient is based on the third estimation when the longitudinal acceleration is not available.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. provisional Application No.61/695,886 filed Aug. 31, 2012, the disclosure of which is incorporatedin its entirety by reference herein.

TECHNICAL FIELD

One or more embodiments relate to a vehicle system and method forestimating road gradient and vehicle mass.

BACKGROUND

Road grade and vehicle mass are two parameters that affect a number ofvehicle control functions. Such functions include powertrain control,energy management control, transmission control, brake control andstability control. For example, conventional vehicles include aninternal combustion engine which provides torque for propelling thevehicle. The engine provides torque through the transmission to thedrive wheels, even when the engine is idling. Such torque is referred toas creep-torque. When a vehicle is stopped on an inclined surface, thiscreep-torque supplements torque applied by a brake system (brake torque)to maintain the vehicle in a stationary position. If the engine were toshutdown, while the vehicle was stopped on the incline, the enginecreep-torque would be eliminated and additional brake torque would beneeded to maintain the vehicle position and prevent the vehicle fromrolling backwards. The amount of additional brake torque needed is basedon road grade and vehicle mass.

A micro-hybrid vehicle may enable a stop/start strategy for starting andstopping the vehicle engine during a driving event. The engine isshutdown if no power is required (e.g. while waiting at a trafficlight). As soon as power is requested, the engine is automaticallyrestarted. By avoiding unnecessary engine idling, the vehicle's fueleconomy will be improved. A micro-hybrid having start/stop functionalitymay adjust brake pressure to maintain a vehicle position when the engineis shutdown while the vehicle is stopped on an incline. Such amicro-hybrid estimates road grade and vehicle mass, and adjusts brakepressure based on these estimates.

SUMMARY

In one embodiment, a vehicle is provided with a controller that isconfigured to generate output indicative of a road gradient based on atleast one of a first estimation, a second estimation and a thirdestimation. The road gradient is based on the first estimation when avehicle speed is less than a speed threshold and an input indicative ofa longitudinal acceleration is available. The road gradient is based onthe second estimation when the vehicle speed is greater than the speedthreshold and the longitudinal acceleration is available. The roadgradient is based on the third estimation when the longitudinalacceleration is not available.

In another embodiment, a vehicle system is provided with a controllerthat is configured to generate output indicative of a road gradientbased on at least one of a static estimation, a kinematic estimation anda dynamic estimation. The road gradient is based on the staticestimation when a vehicle speed is less than a speed threshold. The roadgradient is based on the kinematic estimation when the vehicle speed isgreater than the speed threshold. The road gradient is based on thedynamic estimation when a longitudinal acceleration is not available.The controller is also configured to generate output that is indicativeof a quality factor corresponding to a selected estimation.

In yet another embodiment, a method for arbitrating road gradientestimation is provided. Output is generated that is indicative of a roadgradient based on at least one of a static estimation when a vehiclespeed is less than a speed threshold, a kinematic estimation when thevehicle speed is greater than the speed threshold, and a dynamicestimation when a longitudinal acceleration is not available. Output isalso generated that is indicative of a quality factor corresponding to aselected estimation.

As such the vehicle system provides advantages over existing systems byarbitrating or selecting a road gradient estimation from a number ofdifferent estimations (static, kinematic, and dynamic). Each roadgradient estimation is suited for different vehicle conditions. Byarbitrating between these different estimations, the vehicle systemprovides a road gradient estimation over a wide range of vehicleconditions.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram of a vehicle having a vehicle system forestimating road gradient and vehicle mass according to one or moreembodiments;

FIG. 2 is a side view of the vehicle of FIG. 1 illustrated on a roadinclined at a gradient;

FIG. 3 is a bottom view of the vehicle of FIG. 1;

FIG. 4 is a schematic block diagram illustrating a method forarbitrating road gradient estimation, as performed by the vehicle systemof FIG. 1, according to one or more embodiments;

FIGS. 5A-5E are time plots illustrating road gradient estimationscontrolled by the method of FIG. 4;

FIG. 6 is a flow chart illustrating a method for arbitrating roadgradient estimations according to one or more embodiments;

FIG. 7 is a time plot illustrating vehicle speed and acceleration;

FIG. 8 is a flow chart illustrating a method for evaluatingaccelerometer convergence status according to one or more embodiments;

FIG. 9 is a flow chart illustrating a method for locking the static roadgradient output according to one or more embodiments;

FIGS. 10A-10C are time plots illustrating vehicle speed, accelerationand road gradient estimations as controlled by the method of FIG. 9;

FIG. 11 a schematic block diagram illustrating a method for kinematicroad gradient estimation according to one or more embodiments;

FIG. 12 is a flow chart illustrating a method for estimating kinematicroad gradient according to one or more embodiments;

FIG. 13A-13D are time plots illustrating vehicle speed, acceleration androad gradient estimations as controlled by the method of FIG. 12;

FIG. 14 is a flow chart illustrating a method for determining a vehicledriving situation according to one or more embodiments;

FIG. 15 is a flow chart illustrating a method for estimating a vehiclelateral velocity according to one or more embodiments;

FIG. 16 is a flow chart illustrating a method for estimating a body rollangle according to one or more embodiments;

FIG. 17 is a diagram illustrating a method for selecting suspensionparameters, according to one or more embodiments;

FIG. 18 is a schematic block diagram illustrating a method for vehiclemass estimation according to one or more embodiments;

FIG. 19 is a flow chart illustrating a method for estimating the vehiclemass according to one or more embodiments;

FIG. 20 is a flow chart illustrating a method for resetting vehicle massestimation parameters according to one or more embodiments;

FIG. 21 is a flow chart illustrating a method for updating vehicle massestimation parameters according to one or more embodiments;

FIG. 22 is a time plot illustrating vehicle mass estimations;

FIG. 23 is another time plot illustrating vehicle mass estimations;

FIG. 24 is yet another a time plot illustrating vehicle massestimations;

FIG. 25 is a flow chart illustrating a method for estimating the qualityof an estimator according to one or more embodiments;

FIGS. 26A-26C are time plots illustrating vehicle speed, accelerationand road gradient estimations as controlled by the method of FIG. 25;and

FIGS. 27A-27G are time plots illustrating vehicle speed, and qualityfactors as controlled by the method of FIG. 25.

DETAILED DESCRIPTION

As required, detailed embodiments of the present invention are disclosedherein; however, it is to be understood that the disclosed embodimentsare merely exemplary of the invention that may be embodied in variousand alternative forms. The figures are not necessarily to scale; somefeatures may be exaggerated or minimized to show details of particularcomponents. Therefore, specific structural and functional detailsdisclosed herein are not to be interpreted as limiting, but merely as arepresentative basis for teaching one skilled in the art to variouslyemploy the present invention.

With reference to FIG. 1, a vehicle system for estimating road gradientand vehicle mass is illustrated in accordance with one or moreembodiments and is generally referenced by numeral 10. The vehiclesystem 10 is depicted within a vehicle 12. The vehicle system 10includes a controller, such as an engine control module (ECM) 14, aninternal combustion engine (ICE) 16 and a vehicle system controller(VSC) 18 that are in communication with each other. The VSC 18 receivesinput that corresponds to vehicle speed (Vx), acceleration (a), yaw rate(r) and torque (T_(pwt), T_(brk)), and then provides output thatcorresponds to a road gradient estimation (RGE) and estimated vehiclemass (Mv) in response to the input.

The illustrated embodiment depicts the vehicle 12 as a micro-hybridvehicle, which is a vehicle that is propelled by the engine 16, and theengine 16 is repeatedly started and stopped to conserve fuel. Anenhanced starter motor 20 is coupled to an engine crankshaft. Thestarter motor 20 receives electrical power and provides output torque tothe crankshaft for starting the engine 16. Although illustrated anddescribed in the context of a micro-hybrid vehicle 12, it is understoodthat embodiments of the vehicle system 10 may be implemented on othertypes of vehicles, such as a conventional powertrain vehicle, a batteryelectric vehicle (BEV), which is powered by an electric motor withoutassistance of an engine, and a hybrid electric vehicle (HEV), which ispowered by an electric motor and an engine.

The vehicle 12 includes a transmission 22 for adjusting the outputtorque of the engine 16. Torque from the engine 16 is transferredthrough the transmission 22 to a differential 24 by a transmissionoutput shaft 26. Axle half shafts 28 extend from the differential 24 toa pair of driven wheels 30 to provide drive torque for propelling thevehicle 12.

The vehicle 12 includes a shifter 32 for manually selecting atransmission gear. The shifter 32 includes a sensor (not shown) forproviding an output signal that corresponds to a selected transmissiongear (e.g., PRNDL). A transmission control module (TCM) 34 communicateswith the shifter 32 and the transmission 22 for adjusting thetransmission gear ratio based on the shifter selection. Alternativelythe shifter 32 may be mechanically connected to the transmission 22 foradjusting the transmission gear ratio.

The vehicle 12 includes a braking system which includes a brake pedal36, and a booster and a master cylinder which are generally referencedby brake activation block 38 in FIG. 1. The braking system also includesan ABS brake control module 40 that is connected to wheel brakeassemblies 42 and the brake activation block 38 by a series of hydrauliclines 44 to effect friction braking. The wheel brake assemblies 42 areoriented at each wheel 30 and may be configured as caliper, or drumbrake assemblies. Each wheel brake assembly 42 includes a sensor (notshown) for providing a wheel speed signal (w_(s)). In one or moreembodiments, the ABS brake control module 40 provides a calculatedvehicle speed signal (Vx) to the VSC 18 based on the wheel speedsignals.

The braking system also includes sensors for providing information thatcorresponds to current brake characteristics. The braking systemincludes a position switch for providing a brake pedal state (S_(bp))signal that corresponds to a brake pedal position (e.g., applied orreleased). In other embodiments, the braking system includes a positionsensor (not shown) for measuring pedal position. The braking system alsoincludes one or more pressure sensors for providing a brake pressure(P_(brk)) signal that corresponds to an actual brake pressure valuewithin the brake system (e.g., brake line pressure, master cylinderpressure, or pressure at the wheel assemblies).

The vehicle 12 includes an accelerator pedal 48 with a position sensorfor providing an accelerator pedal position (APP) signal thatcorresponds to a driver request for propulsion. The ECM 14 controls thethrottle of the engine 16 based on the APP signal.

The vehicle 12 includes an energy storage device, such as a battery 50.The battery 50 supplies electrical energy to the vehicle controllers andthe starter motor 20, as generally indicated by dashed lines in FIG. 1.The vehicle 12 may include a single battery 50, such as a conventionallow voltage battery, or multiple batteries, including a high voltagebattery. Additionally, the vehicle 12 may include other types of energystorage devices, such as capacitors or fuel cells.

The vehicle 12 also includes one or more inertial sensors 52 whichprovide output that corresponds to vehicle acceleration (a). In oneembodiment, the inertial sensor 52 provides output corresponding tolongitudinal acceleration. In another embodiment, the inertial sensors52 provide output corresponding to longitudinal acceleration, lateralacceleration and vertical acceleration, all of which are generallyreferenced by the variable “a” in FIG. 1. In one or more embodiments,the inertial sensors 52 also provide output (r) corresponding to a yawrate. In other embodiments, the sensor 52 is an inclinometer (not shown)which provides output corresponding to a road gradient combined with avehicle pitch angle.

The vehicle 12 includes a navigation system 54 for receiving destinationinformation from a driver. The navigation system 54 also communicateswith a plurality of systems/networks that are external to the vehicle.For example the navigation system 54 may include transceivers forcommunicating with satellites (e.g. GPS), and computers (via cellularnetworks, hardwire connections or radio frequency (RF) waves) (notshown). Such networks are described in detail in U.S. application Ser.No. 12/744,092 to Pleet et al., which is hereby incorporated byreference. The navigation system 54 determines a distance between apresent location of the vehicle 12 and the destination selected by thedriver, and provides this distance, along with speed, acceleration andelevation information associated with the vehicle 12, its targetdestinations, or other relevant GPS waypoints to the VSC 18.

The VSC 18 communicates with other vehicle systems, sensors andcontrollers for coordinating their function. As shown in the illustratedembodiment, the VSC 18 receives a plurality of input signals (e.g.,S_(bp), P_(brk), engine speed (Ne), vehicle speed (Vx), powertraintorque (T_(pt)), brake torque (T_(br)), a, and r) from various vehiclesensors and controllers. Although it is shown as a single controller,the VSC 18 may include multiple controllers that may be used to controlmultiple vehicle systems according to an overall vehicle control logic,or software. The vehicle controllers, including the VSC 18 generallyinclude any number of microprocessors, ASICs, ICs, memory (e.g., FLASH,ROM, RAM, EPROM and/or EEPROM) and software code to co-act with oneanother to perform a series of operations. The controllers also includepredetermined data, or “look up tables” that are based on calculationsand test data and stored within the memory. The VSC 18 communicates withother vehicle systems and controllers (e.g., the ECM 14, the TCM 34,etc.) over one or more wired or wireless vehicle connections usingcommon bus protocols (e.g., CAN and LIN).

With reference to FIGS. 2 and 3, the vehicle 12 is illustrated with anumber of forces, accelerations and moments acting thereon. The vehicle12 is shown with reference to a coordinate system centered about acenter of gravity of the vehicle (CG). The coordinate system includes alongitudinal axis (x), a lateral axis (y) and a vertical axis (z).Referring to FIG. 2, the vehicle 12 is positioned upon an inclinedsurface. The surface has a gradient (road gradient) that is representedby the symbol α_(r). Additionally, the vehicle pitch angle relative tothe road gradient is represented by symbol θ. The weight of the vehicleis represented by Mg, which is illustrated acting upon the CG of thevehicle 12, where M is estimated vehicle mass, and g represents theacceleration due to gravity (9.8 m/s²). The longitudinal velocity of thevehicle 12 is represented by symbol V_(x), and is determined based onthe wheel speed signals (W_(s) shown in FIG. 1), according to one ormore embodiments. The longitudinal acceleration and the verticalacceleration of the vehicle 12 are represented by variables a_(x) ^(s),a_(z) ^(s) respectively, where the superscript letter(s) indicates thatthe signal is provided by a sensor, such as the inertial sensors 52shown in FIG. 1. Referring to FIG. 3 the yaw angular rate acts about thevertical axis (z) and is represented by letter r. The inertial sensor(s)52 are mounted on the vehicle 12 offset from the CG, and the vehiclesystem 10 calculates acceleration values at the CG based on the offset.The longitudinal offset between the sensors 52 and the CG is representedby l_(d), and the lateral offset is represented by l_(c) in FIG. 3.

FIG. 4 depicts a simplified block diagram of a method 410 for roadgradient estimation arbitration in accordance with one or moreembodiments. The method is implemented using software code containedwithin the VSC, according to one or more embodiments. In otherembodiments, the method 410 is implemented in other vehicle controllers,or distributed amongst multiple vehicle controllers. The vehicle systemincludes a number of algorithms for estimating road gradient, where eachalgorithm is suited for different vehicle conditions. The method 410selects a road gradient estimation algorithm based on current vehicleconditions.

The VSC receives inputs at signal processing block 412. The inputsinclude vehicle states and sensor signals, (e.g., a_(x) ^(s), a_(y)^(s), a_(z) ^(s), r) derived or calculated values (e.g., Vx, T_(pwt),T_(brk), F_(pwt), F_(brk)), and quality factors which are all generallyrepresented by input 414. In some embodiments, the vehicle systemreceives inputs from the navigation system that correspond to vehiclespeed and acceleration, which are generally represented by input 418.

At block 420 the VSC estimates road gradient (RGE) using static,kinematic and dynamic estimation algorithms, which are represented bysub-blocks 422, 424 and 426 respectively. The RGE values are provided toan RGE arbitration and diagnostic block 428 which selects whichestimation (RGE_(st), RGE_(kin), or RGE_(dyn)) to provide as an output(RGE_(out)) for other vehicle systems based on a number of vehicleconditions.

The static RGE algorithm 422 estimates road gradient based on thelongitudinal acceleration input (a_(x) ^(s)). Static RGE is suited forvehicle standstill conditions, such as engine start/stop functionalitywhen a micro-hybrid is stopped on an incline. When the vehicle isstopped on an incline (as shown in FIG. 2) the longitudinal accelerationvalue (a_(x) ^(s)) is biased due to deflection within the vehiclesuspension system, which is represented by the vehicle pitch angle (θ).The static RGE algorithm compensates for the vehicle pitch angle, asdescribed in detail with respect to FIGS. 7-10C. Generally, the staticRGE is the most accurate algorithm among the three RGE estimations sinceits estimation involves the minimal estimation input and it is the mostdirectly derived estimation results. However, the static RGE is onlyavailable at vehicle standstill conditions.

The kinematic RGE algorithm 424 estimates RGE based on inputcorresponding to the vehicle's kinematic properties including therelationship between vehicle speed, accelerations and yaw rate. Thealgorithm processes the input using a Kalman Filter as described indetail with respect to FIGS. 11-13D. The kinematic RGE algorithm 424 issuited for normal vehicle motion conditions (e.g., Vx>5 kph) becausesome of the underlying calculations have low accuracy at low vehiclespeeds. Additionally, a Kalman filter is a dynamic filter, a goodestimation is usually not available until the estimation state hasconverged. The transient time depends on the difference between theinitial state and the true road grade state as well as the estimationconditions like: signal frequency component richness and signal to noiseratio, etc.

The dynamic RGE algorithm 426 estimates RGE based on input correspondingto vehicle speed, wheel torques (brake torque and powertrain outputtorque), and other drag forces acting on the vehicle (e.g., aerodynamicdrag, rolling resistance and the road gradient load). The vehicle systemincludes an algorithm for estimating vehicle mass, which is included inblock 426, and described in detail with respect to FIGS. 18-24.Generally the dynamic RGE has lower quality conditions as compared tostatic and kinematic RGE because it involves more signals in itsestimation, and the noise and error in each signal will be propagated inthe final RGE output. However, the dynamic RGE does not rely on thelongitudinal acceleration signal (a_(x) ^(s)), and therefore it hasdifferent signal coverage from that of the static and the kinematicestimations. For example, in the case of sensor error conditions thelongitudinal acceleration signal (a_(x) ^(s)) may not be available, andthen the dynamic RGE is the only available estimate.

At block 430 the vehicle system performs driving state monitoring andestimation algorithm adjustment. The estimation algorithm adjustmentincludes feedforward input compensation for the RGE_(kin) algorithm ofblock 424, which is described in detail with respect to FIGS. 14-17.

At block 432 the vehicle system evaluates vehicle operation conditions,such as vehicle stability, oscillation and speed, and provides acorresponding judgment to block 428. At block 434 the vehicle systemdetermines a quality factor for each RGE algorithm (QF_(st), QF_(kin),QF_(dyn)), along with an overall RGE quality factor (QF_(RGE)). In oneor more embodiments the QF_(RGE) is provided with the RGE_(out) to othervehicle systems. Such quality evaluations are described in detail withrespect to FIGS. 25-27G. Generally, a quality factor evaluates both theavailability and the accuracy of the estimated information. The qualityfactor is selected from a range of 0-3, where a quality value of three(3) indicates full quality, two (2) indicates some degradation, (1)indicates poor quality, and a value of zero indicates that estimation isnot currently available.

FIGS. 5A-5E illustrate the impact of the method 410 for arbitrating roadgradient estimations of FIG. 4. FIGS. 5A-5E include 5 graphs ofmeasurement data taken over a common period of time on a road having aknown actual road gradient. FIG. 5A illustrates vehicle speed (Vx) andthe static road gradient estimation (RGE_(st)). FIG. 5B illustratesvehicle speed (Vx) and the kinematic road gradient estimation(RGE_(out)). FIG. 5C illustrates vehicle speed (Vx) and the roadgradient estimation output (RGE_(out)). FIG. 5D illustrates the roadgradient estimation output (RGE_(out)) and the actual road gradient(α_(r)). FIG. 5E illustrates the percent error between RGE_(out) andα_(r).

FIG. 5A-5C illustrate how the method 410 arbitrates between the staticand kinematic road gradient estimations during a drive cycle where thevehicle makes a number of stops. Vehicle speed (Vx) is illustrated oneach of FIGS. 5A-5C. The vehicle makes three stops in the illustratedembodiment, which are referenced by numerals 512, 514 and 516.

As depicted in FIG. 5A, the static road gradient estimate (RGE_(st)) isavailable when the vehicle stops, or is at standstill. At the first stop512, the RGE_(st) indicates that the road gradient is approximately 20%,as represented by numeral 518. The RGE_(st) indicates that the roadgradient is approximately 0% at the second stop, and −12% at the thirdstop, as indicated by numerals 520 and 522 respectively. When thevehicle is moving, static road gradient estimation is not available, andthe RGE_(st) waveform is equal to zero, according to one or moreembodiments.

As depicted in FIG. 5B, the kinematic road gradient estimate (RGE_(kin))is available when the vehicle is moving or during normal vehicle motion.Prior to the first stop 512, the RGE_(kin) indicates that the roadgradient increases from approximately 0% to 20%, as represented bynumeral 524. At the first stop 512, RGE_(kin) is not available, andRGE_(kin) latches/locks to the last RGE_(kin) value as represented bynumeral 526. Between the first stop 512 and the second stop 514, theRGE_(kin) decreases from approximately 20% to 0% as represented bynumeral 528. Between the second stop 514 and the third stop 516, theRGE_(kin) decreases from approximately 0% to −12% as represented bynumeral 530. After the third stop 516, the RGE_(kin) increases from −12%to 0%, as represented by numeral 532.

As depicted in FIG. 5C, the output road gradient estimate (RGE_(out)) asdescribed with reference to FIG. 4, generally selects RGE_(st) atstandstill conditions and RGE_(kin) when the vehicle is moving. FIG. 5Dillustrates RGE_(out) along with the actual road gradient (α_(r)). FIG.5E depicts the percent error (Error) between RGE_(out) and α_(r). Forthis example, the error is approximately +/−4%.

Table A depicts a comparison of the inputs and vehicle conditionsassociated with the algorithms for estimating road gradient, and isshown below:

Estimation Input Estimation Availability RGE Vehicle Wheel Torque VeryLow Normal Convergence Algorithms Speed Acceleration & Force StandstillSpeeds Motion Phase Static X X  Yes* Kinematic X X X Yes Dynamic X X X XYes

Column 1 of Table A illustrates the three road gradient estimationalgorithms: static, kinematic and dynamic. Columns 2-4 depict theprimary inputs that each algorithm uses for its estimate. For example,the vehicle system estimates a static road gradient based on vehicleacceleration. The vehicle system estimates a kinematic road gradientbased on vehicle speed and acceleration, and the vehicle systemestimates a dynamic road gradient based on vehicle speed and wheeltorque/force. These inputs may be measured values, or derived values asdescribed with reference to FIG. 1. Additionally, in some embodiments,the vehicle system uses inputs in addition to those depicted in FIG. 5,for the corresponding road gradient estimates.

Columns 5-8 depict the vehicle conditions at which each road gradeestimate is available. For example, the static road gradient estimationis available when the vehicle is at standstill. The kinematic roadgradient estimation is available when the vehicle is in normal motion,(e.g., vehicle speed is greater than 5 mph). The dynamic road gradientestimation is available when the vehicle is at standstill, or duringnormal vehicle motion. Although column 6 of the table indicates thatnone of the estimates are available at very low vehicle speeds, (e.g.,less than 5 mph), the vehicle system still provides an accurate roadgradient estimate during such conditions, as described below withreference to FIGS. 25-26C.

Column 7 depicts the convergence phase of each estimate. The convergencephase relates to the amount of time it takes the vehicle system toestimate a road gradient using each algorithm during a transitionbetween vehicle conditions (e.g., from standstill to very low speed, tonormal motion).

With reference to FIG. 6, a method for arbitrating road gradientestimations is illustrated in accordance with one or more embodimentsand is generally referenced by numeral 610. The method is implementedusing software code contained within the VSC, according to one or moreembodiments. In other embodiments, the method 610 is implemented inother vehicle controllers, or distributed amongst multiple vehiclecontrollers. In one or more embodiments, the operations of the method610 correspond to the operations of method 410 described with referenceto FIG. 4. The vehicle system includes a number of algorithms forestimating road gradient, where each algorithm is suited for differentvehicle conditions. The method 610 selects a road gradient estimationalgorithm based on estimation status, estimation readiness andestimation quality factors.

The estimation status factor for each RGE algorithm (static, kinematic,and dynamic) is defined as a Boolean variable (TRUE/FALSE) andrepresented by variables (STATUS_(st), STATUS_(kin), and STATUS_(dyn)).The estimation status factor indicates whether the estimation conditionis satisfied and if the estimation function is updating its state at acorresponding RGE channel (RGE_(st), RGE_(kin), and RGE_(dyn)). A statusvalue of TRUE indicates that the estimation method is activelymonitoring the vehicle dynamic states (e.g., Vx, a_(x) ^(s), a_(y) ^(s),a_(z) ^(s), and r) and the road gradient information and its estimationchannel are updating.

The estimation readiness factor for each RGE algorithm (static,kinematic, and dynamic) is also defined as a Boolean variable(TRUE/FALSE) and represented by variables (READY_(st), READY_(kin), andREADY_(dyn)). The estimation readiness generally indicates the presentusefulness of the RGE function. More specifically, the static estimationreadiness (READY_(st),) indicates whether RGE_(st) has already provideda road grade estimate for the present vehicle location. The kinematicand dynamic estimation readiness values (READY_(kin), and READY_(dyn))indicate that their estimation state is validated with quality factorgreater than 2 (degraded, or full quality) for more than a thresholdperiod of time. In the presence of an unqualified estimation condition,the accumulated time under such condition has not exceeded another timethreshold such that the estimation state has not deviated out of thedegraded boundary.

The estimation quality factor for the RGE output as well as that foreach RGE algorithm (static, kinematic, and dynamic) corresponds to avalue between 0-3, and is represented by variables (QF_(RGE), QF_(st),QF_(kin), and QF_(dyn)). The estimation quality factor evaluates boththe availability and the accuracy of the estimated road gradientinformation. QF_(RGE) is the general RGE output signal qualityevaluation, which is based on the quality factor of the selected RGEalgorithm, and further processed according to the vehicle's dynamicsituation after the update event. Such quality evaluation is describedin detail with respect to FIGS. 25-26C. For the evaluated quality factorfrom individual channels, QF_(st), QF_(kin), and QF_(dyn), a value of 3indicates full quality and a value of 2 indicates that estimation isstill available but it is possible that the current value is outside ofthe specified precision range.

At operation 612, the vehicle system evaluates the current estimationstatus of the static RGE function to determine if STATUS_(st) is equalto TRUE. If the determination at operation 612 is positive, the vehiclesystem proceeds to operation 614. At operation 614, the vehicle systemevaluates the current estimation readiness of the static RGE function todetermine if READY_(st) is equal to TRUE. If the determination atoperation 614 is positive, the vehicle system proceeds to operation 616.At operation 616, the vehicle system evaluates the current estimationquality of the static RGE function to determine if QF_(st) is equal tothree (full quality). If the determination at operation 616 is positive,the vehicle system proceeds to operation 618, and selects the staticroad gradient estimation by setting the RGE output equal to RGE_(st). Ifthe determination at operation 612 is negative, the vehicle systemproceeds to operation 620.

At operation 620, the vehicle system evaluates the current estimationstatus of the kinematic RGE function to determine if STATUS_(kin) isequal to TRUE. If the determination at operation 620 is positive, thevehicle system proceeds to operation 622. At operation 622, the vehiclesystem evaluates the current estimation readiness of the kinematic RGEfunction to determine if READY_(kin) is equal to TRUE. If thedetermination at operation 622 is positive, the vehicle system proceedsto operation 624. At operation 624, the vehicle system evaluates thecurrent estimation quality of the kinetic RGE function to determine ifQF_(kin) is equal to three (full quality). If the determination atoperation 616 is positive, the vehicle system proceeds to operation 626and selects the kinematic road gradient estimation by setting the RGEoutput equal to RGE_(kin).

The static and kinematic road gradient estimations are preferred overRGE_(dyn) because RGE_(dyn) has lower quality conditions due to theadditional signals in its estimation, and the noise in each signal willbe propagated in the final RGE output. Therefore even if the static andkinematic quality factors are less than three, the vehicle system maystill select RGE_(st) or RGE_(kin) over RGE_(dyn).

If the determination at operation 616 is negative, then QF_(st) is lessthan 3, and the vehicle system proceeds to operation 628. At operation628 the vehicle system sets a temporary quality factor value (QF_(tmp))equal to QF_(st), and a temporary road gradient estimate (RGE_(tmp))equal to RGE_(st). Similarly, if the determination at operation 624 isnegative, then QF_(kin) is less than 3, and the vehicle system proceedsto operation 630. At operation 630 the vehicle system sets a temporaryquality factor value (QF_(tmp)) equal to QF_(kin), and a temporary roadgradient estimate (RGE_(tmp)) equal to RGE_(kin). After both operations628 and 630, the vehicle system proceeds to operation 632 to evaluatethe overall RGE quality factor (QF_(RGE)).

At operation 632 the vehicle system determines if QF_(RGE) is less than3. If the determination is negative, the vehicle system proceeds tooperation 634 and does not update RGE. If the overall RGE has fullquality, the vehicle system is not going to select a different RGE oflesser quality. If the determination at operation 632 is negative, thenthe vehicle system proceeds to operation 636.

At operation 636, the vehicle system evaluates the current estimationstatus and the current estimation readiness of the dynamic RGE functionto determine if both STATUS_(dyn) and READY_(dyn) are equal to TRUE. Ifthe determination at operation 636 is negative, the vehicle systemproceeds to operation 638. At operation 638 the vehicle system evaluatesthe temporary quality factor QF_(tmp) to determine if it is greater thanor equal to 2. If the determination at operation 638 is negative, thevehicle system proceeds to operation 634 and does not update RGE. If thedetermination at operation 638 is positive, the vehicle system proceedsto operation 640 and selects the temporary road gradient (static orkinematic) by setting RGE equal to RGE_(tmp). If the determination atoperation 636 is positive, the vehicle system proceeds to operation 642.

At operation 642, the vehicle system evaluates the temporary qualityfactor (QF_(tmp)) and the dynamic quality factor (QF_(dyn)), todetermine if the minimum value of QF_(tmp) and QF_(dyn) is greater thanor equal to 2. If the determination at operation 642 is positive, thevehicle system proceeds to operation 644. At operation 644 the vehiclesystem determines if QF_(tmp) is greater than QF_(dyn). If thedetermination at operation 644 is positive, the vehicle system proceedsto operation 640 and selects the temporary road gradient (static orkinematic) by setting RGE equal to RGE_(tmp). If the determination atoperation 644 is negative (QF_(tmp) is not greater than QF_(dyn)), thenthe vehicle system proceeds to operation 646 and selects the dynamicroad gradient by setting RGE equal to RGE_(dyn).

If the determination at operations 614, 620 or 622 is negative, thevehicle system proceeds to operation 648. At operation 648, the vehiclesystem evaluates the current estimation status and the currentestimation readiness of the dynamic RGE function to determine if bothSTATUS_(dyn) and READY_(dyn) are equal to TRUE.

If the determination at operation 648 is positive, the vehicle systemproceeds to operation 650. At operation 650, the vehicle systemevaluates the current estimation quality of the dynamic RGE function todetermine if QF_(dyn) is greater than or equal to two. If thedetermination at operation 650 is positive, the vehicle system proceedsto operation 646 and selects the dynamic road gradient estimation bysetting the RGE output equal to RGE_(dyn). If the determination atoperations 648 or 650 are negative, the vehicle system proceeds tooperation 652 and does not update the RGE output, by latching the RGEoutput value to the previous RGE output value. After operations 618,626, 640, 646, and 652 the vehicle system proceeds to operation 654. Atoperation 654 the vehicle system provides the RGE output and QF, andthen returns to operation 612 for another iteration of the method 610.

As such the vehicle system provides advantages over existing systems byarbitrating or selecting a road gradient estimation from a number ofdifferent estimations (static, kinematic, and dynamic). Each roadgradient estimation is suited for different vehicle conditions. Byarbitrating between these different estimations, the vehicle systemprovides a road gradient estimation over a wide range of vehicleconditions.

Referring back to FIG. 2, the static and kinematic road grade estimatesare based on vehicle kinematic equations. Kinematics generally refers todescription of how a body moves in space and time. The vehicle kinematicequations are based on the vehicle acceleration values (a_(x) ^(s),a_(y) ^(s), a_(z) ^(s)) that are provided by the inertial sensors (52).The coordinate system (x, y, z) is oriented relative to the road andabout the center of gravity (CG) of the vehicle. However, the inertialsensors 52 are mounted offset from the coordinate system (x, y, z),where l_(c) is a lateral offset and l_(d) is a longitudinal offset.Additionally, the inertial sensors 52 provide output signals that areoffset from the coordinate system due to the vehicle suspension. Thevehicle pitch angle (θ) represents an angular offset from thelongitudinal (x) axis. The vehicle roll angle (ϕ) (not shown) representsan angular offset from the lateral (y) axis, and the vehicle yaw rate(r) represents an angular offset from the vertical (z) axis. The vehiclesystem estimates the road gradient (α_(r)), and therefore separates thevehicle offsets (e.g., vehicle pitch angle (θ)) from the road gradient(α_(r)).

The governing kinematic vehicle frame equations are represented below:a _(x) ^(s) +l _(c) {dot over (r)}=({dot over (V)} _(x) −r*V _(y))cosθ+g sin(α_(r)+θ)  (1)a _(y) ^(s) +l _(d) {dot over (r)}=({dot over (V)} _(y) +r*V _(x))cosϕ_(v) −g sin ϕ cos(α_(r)+θ)  (2)a _(z) ^(s) ={dot over (V)} _(x) sin θ+g cos(α_(r)+θ)+n _(az)  (3)where a_(x) ^(x), a_(y) ^(s), a_(z) ^(s) are the vehicle longitudinal,lateral and vertical accelerations. l_(c) is the sensor lateral offsetdisplacement form the vehicle rotational center (CG) in a turningmaneuver, and l_(d) is the sensor longitudinal offset displacement.V_(x), and V_(y), are longitudinal speed and lateral speed; and {dotover (V)}_(x), {dot over (V)}_(y), and {dot over (V)}_(z), are thecalculated longitudinal, lateral and vertical accelerations; θ is thevehicle pitch angle and r is the yaw rate. ϕ is the total vehicle rollangle, which corresponds to the sum of the vehicle body roll angle ϕ_(v)and the road bank angle ϕ_(b). Additionally, n_(az) represents all ofthe vertical acceleration disturbances caused by vehicle suspensionoscillation and road surface fluctuation.

The vehicle system estimates a static road gradient based on the vehiclelongitudinal acceleration and equation 1. Since the vehicle is static,or at standstill, the longitudinal velocity (V_(y)), the longitudinalacceleration ({dot over (V)}_(x)), yaw rate, and the derivative of theyaw rate are set to zero, and the kinematic equations are simplified.Equation 4 illustrates an equation for calculating longitudinalacceleration at static conditions, and is shown below:a _(x) ^(s) =g(α_(r)+θ)≈g sin α_(r) +gθ  (4)

The vehicle pitch angle (θ) due to suspension deflection is proportionalto the road gradient angle as θ≈K_(p)(g sin α_(r)) where K_(p)represents the stiffness of the vehicle body at steady state. K_(p) is apredetermined value based on the magnitude of a_(x) ^(s). In one or moreembodiments, the VSC includes stored data corresponding to a lookuptable of pre-calibrated data with the value of K_(p) interpolated withrespect to the vector of a_(x) ^(s) values across its positive andnegative operating range. Equation 5 illustrates an intermediatederivation step, and equation 6 provides an equation for calculating theroad gradient (α_(r)) at static conditions, as shown below:

$\begin{matrix}{a_{x}^{s} \approx {{g\left( {1 + {K_{p} \cdot g}} \right)}\sin\;\alpha_{r}}} & (5) \\{\alpha_{r} = {a\;{\sin\left( \frac{a_{x}^{s}}{g\left( {1 + {K_{p}g}} \right)} \right)}}} & (6)\end{matrix}$

Generally, the static RGE is the most accurate algorithm among the threeRGE estimations since it involves the minimal estimation input and it isthe most directly derived estimation results. However, the static RGE isonly available at vehicle standstill conditions.

FIG. 7 illustrates measurement data taken as the vehicle stops on agrade, according to one or more embodiments. FIG. 7 illustrates inputsignals vehicle speed (Vx), and longitudinal acceleration (a_(x) ^(s)),along with road grade (α_(r)) data calculated according to equation 6.At time t₁, the vehicle is decelerating as indicated by the decreasingslope of Vx. At time t₂, the vehicle stops. After the vehicle stops,there is noise present on the longitudinal acceleration signal (a_(x)^(s)) due to vehicle body oscillations, as referenced by numeral 712.This noise is amplified by the road gradient calculation, as illustratedby the road gradient α_(r) waveform, and referenced by numeral 714. Attime t₃, the longitudinal acceleration signal (a_(x) ^(s)) has convergedenough to provide quality information for road grade estimation, asreferenced by numeral 716. The time difference between t₂ and t₃ isapproximately 3 s in the illustrated embodiment. Therefore a vehiclesystem relying on the road gradient (α_(r)) calculation for RGE, maywait approximately 3 s before using the value. This time delay resultsin decreased fuel efficiency for a micro-hybrid utilizing enginestart/stop functionality.

With reference to FIG. 8, a method for evaluating accelerometerconvergence status is illustrated in accordance with one or moreembodiments and is generally referenced by numeral 810. The method isimplemented using software code contained within the VSC, according toone or more embodiments. While the vehicle system is estimating thestatic road gradient, its estimation state and quality may becontinuously monitored. High quality estimation occurs when theaccelerometer output (a_(x) ^(s)) is converged and (a_(x) ^(s)) is keptin such a steady state for a minimal amount of time. The accelerometeroutput (a_(x) ^(s)) is assigned a convergence status factor (CON_(st))that indicates whether the raw signal is in steady state or not in threelevel of label code. Where CON_(st) that is equal to 0, indicates thatthe convergence is not stable; a CON_(st) equal to 1 indicates that theconvergence is somewhat stable; and a CON_(st) equal to 2 indicates thatthe convergences is very stable.

Additionally, the convergence status signal (CON_(st)) is also used toevaluate the static RGE output quality. For example, in one or moreembodiments, a CON_(st) of 2 (very stable), corresponds to a highest orfull quality estimation (QF_(st)=3). A CON_(st) of 1 (somewhat stable)corresponds to a degraded quality estimation (QF_(st)=2). A CON_(st) of0 (not stable) corresponds to a non-qualified estimation (QF_(st)=1), orno data (QF_(st)=0). The convergence status signal (CON_(st)) is basedon the derivative of longitudinal acceleration.

At operation 812 the vehicle system evaluates the vehicle speed (Vx) andthe convergence status factor (CON_(st)) to determine if both Vx is lessthan a low speed threshold; and if the accelerometer signal quality isok (e.g., if CON_(st)=2). If the determination at operation 812 isnegative, the vehicle system proceeds to operation 814. At operation 814the vehicle system evaluates the convergence status factor (CON_(st)) todetermine if CONst indicates that the signal is stable (e.g., ifCON_(st)=2). If the determination at operation 814 is negative, thevehicle system proceeds to operation 816 and resets the timers and flagsby setting a stable timer, a converging timer, a nonstable timer, andthe convergence status factor (CON_(st)) all to zero (CON_(st)=NotStable). The vehicle system also sets a Grade_(found) flag equal toFALSE, when there is an RGE value available. Grade_(found) indicateswhether a valid estimation (RGEst) has already been provided for apresent vehicle stop event. A Grade_(found) that is equal to TRUEindicates that a valid RGE_(st) has already been provided, andGrade_(found) that is equal to FALSE indicates that a valid RGE_(st) hasnot already been provided or is no longer valid. If the determination atoperation 812 is positive, the vehicle system proceeds to operation 818.

At operation 818 the vehicle system computes a longitudinal jerk ({dotover (a)}_(x) ^(s)), or a first order derivative of the longitudinalacceleration using a low pass filter. At operation 820, the vehiclesystem compares the jerk ({dot over (a)}_(x) ^(s)) to a convergingthreshold to determine if {dot over (a)}_(x) ^(s) is less than theconverging threshold. If the determination at operation 820 is negative,then the vehicle system proceeds to operation 822 and the nonstabletimer accrues with cycle time Dt. The vehicle system also proceeds tooperation 822 if the determination at operation 814 is positive. Atoperation 824 the vehicle system evaluates the non-stable timer, todetermine if the non-stable timer is greater than a non-stable timerthreshold. If the determination at operation 824 is positive, thevehicle system proceeds to operation 826 and sets the converging timer,the stable timer and the converging status factor (CON_(st)) all to zero(CON_(st)=Not Stable). If the determination at operation 820 ispositive, the vehicle system proceeds to operation 828.

At operation 828 the vehicle system evaluates the longitudinal jerk({dot over (a)}_(x) ^(s)), to determine if {dot over (a)}_(x) ^(s) isless than a stabilizing threshold. If the determination at operation 828is negative, the vehicle system proceeds to operation 830 and theconverging timer accrues with cycle time Dt. At operation 832 thevehicle system evaluates the converging timer to determine if theconverging timer is greater than the converging time threshold. If thedetermination at operation 832 is positive, the vehicle system proceedsto operation 834 and resets the nonstable and stable timers by settingthem equal to zero; sets the converging status factor to somewhat stable(CON_(st)=1); and sets the Grade_(found) to TRUE. If the determinationat operation 828 is positive, the vehicle system proceeds to operation836.

At operation 836 the stable timer accrues with cycle time Dt. Atoperation 838 the vehicle system evaluates the stable timer to determineif the stable timer is greater than a stabilizing time threshold. If thedetermination at operation 838 is positive, the vehicle system proceedsto operation 840 and resets the nonstable and converging timers bysetting them equal to zero; sets the converging status factor to verystable (CON_(st)=2); and sets the Grade_(found) to TRUE.

With reference to FIG. 9, a method for locking the static road gradientoutput is illustrated in accordance with one or more embodiments and isgenerally referenced by numeral 910. The method is implemented usingsoftware code contained within the VSC, according to one or moreembodiments. Generally once the vehicle system determines that theconvergence status is very stable (e.g., CON_(st)=2), then RGE_(st) islocked to avoid any impact from future vehicle disturbances. The lockwill be released when the vehicle system detects that the vehicle ismoving. A STATIC_(lock) signal indicates whether RGE_(st) is locked,unlocked or quarantined. A lock status of quarantined, indicates anintermediate state between lock and unlock where the vehicle system iswaiting for additional information.

At operation 912 the vehicle system enters an initialization state thatsets the STATIC_(lock) signal equal to unlocked. At operation 914, thevehicle system evaluates the STATIC_(lock) signal to determine if thestatic road gradient (RGE_(st)) is locked. If the determination atoperation 914 is negative, the vehicle system proceeds to operation 916.At operation 916 the vehicle system determines if the STATIC_(lock)signal is quarantined. If the determination at 916 is negative, thevehicle system proceeds to operation 918. At operation 918 the vehiclesystem determines if the GRADE_(found) is TRUE. The GRADE_(found) signalis based on the current RGE_(out) signal. If the determination atoperation 918 is positive, the vehicle system proceeds to operation 920.At operation 920 the vehicle system evaluates the convergence statussignal to determine if CON_(st) is “very” stable, (e.g., if CON_(st)=2).If the determination at operation 920 is positive, the vehicle systemproceeds to operation 922 and sets STATIC_(lock) to locked. Then inoperation 924 the static road gradient estimate (RGE_(st)) is locked. Ifthe determination at operation 914 is positive (e.g.,STATIC_(lock)=locked) then the vehicle system proceeds to operation 926.

At operation 926 the vehicle system evaluates the grade found(GRADE_(found)) to determine if GRADE_(found)=TRUE. If the determinationat operation 926 is positive, the vehicle system proceeds to operation928 and evaluates the convergence status signal to determine if CON_(st)is not stable, (e.g., if CON_(st)=0). If the determination at operation928 is positive, the vehicle system proceeds to operation 930 and setsSTATIC_(lock) to quarantined. After operation 930, and if thedetermination at operations 928 or 934 are negative, the vehicle systemproceeds to operation 924 and locks the static road gradient estimate(RGE_(st)). If the determination at operation 916 is positive, thevehicle system proceeds to operation 932.

At operation 932 the vehicle system evaluates the grade found signal todetermine if GRADE_(found)=TRUE. If the determination at operation 932is positive, the vehicle system proceeds to operation 934 and evaluatesthe convergence status signal to determine if CON_(st) is very stable,(e.g., if CON_(st)=2). If the determination at operation 934 ispositive, the vehicle system proceeds to operation 922 and setsSTATIC_(lock) to locked. If the determination at operation 932, 918, 920or 926 is negative, the vehicle system proceeds to operation 936 setsSTATIC_(lock) to locked. After operations 936 and 924, the vehiclesystem proceeds to operation 938, and then returns to operation 914.

With reference to FIGS. 10A-10C, the vehicle system estimates the staticroad gradient (RGEst) based on the methods 810 and 910 of FIGS. 8 and 9.FIGS. 10A-10C include 3 graphs of measurement data taken over a commonperiod of time and are generally referenced by numeral 1010.Additionally, the vehicle system estimates an initial value for RGE_(st)based on either the kinematic or dynamic road gradient estimates, andthen filters the longitudinal acceleration signal (a_(x) ^(s)) using alow pass filter having a variable bandwidth to determine RGE_(st),according to one or more embodiments.

FIG. 10A illustrates vehicle speed (Vx) and a signal representing thevariable bandwidth of the low pass filter (flt) where the filter has anarrow bandwidth when flt is equal to 1, and a wide bandwidth when fltis equal to zero. FIG. 10B illustrate the measured longitudinalacceleration value (a_(x) ^(s)). FIG. 10C illustrates the static roadgrade estimation (RGE_(st)) along with the kinematic road gradeestimation (RGE_(kin)). In one or more embodiments the bandwidths aredistributed along a frequency range between 2.5 Hz and 0.04 Hz; wherethe wide bandwidth corresponds to the high end of the frequency rangeand the narrow bandwidth corresponds to the low end of the frequencyrange. In other embodiments, the wide bandwidth corresponds toapproximately 1 Hz and the narrow bandwidth corresponds to approximately0.05 Hz.

The vehicle system estimates an initial value for RGEst based on eitherthe kinematic or dynamic road gradient estimates. If no estimates areavailable, the vehicle system will use a default value. Since the staticRGE function only starts after the vehicle speed is lower than apredefined speed threshold, (e.g., 1 kph), the initial value of theestimation algorithm is a factor in the convergence time, or the timedelay until RGE_(st) converges to the actual road gradient. Fortunately,the vehicle system includes multiple road gradient algorithms. The otherkinematic and dynamic estimation algorithms primarily work in thecondition while the vehicle is in motion. Thus, before the start of thestatic RGE algorithm, information about the road gradient severalseconds before the current time or several meters after the vehicle(assuming the vehicle is driving forward) shall be available from theother RGE algorithms and they are the valid reference for the static RGEalgorithm to start with. After that, the static RGE algorithm takes overthe main estimation task to continue monitoring the road gradientvariation while the other two algorithms are paused in absence ofqualified estimation conditions. More specifically, the static RGEalgorithm, when estimation conditions satisfy, will take the output ofeither the kinematic or the dynamic estimation algorithm as its initialvalue depending on which one has the highest quality evaluation. Whenneither of them has satisfied quality, a default value will be usedinstead. The qualified static estimation condition is that the vehiclespeed will be lower than a speed threshold and such speed threshold willbe smaller than the speed thresholds that used to determine on-hold ofthe other two RGE algorithms.

At time t₁, the vehicle starts decelerating as indicated by thedecreasing slope of Vx. At time t₂, the vehicle stops and the RGE_(st)becomes active. Rather than starting from 0%, and converging to theactual road gradient, the RGE starts from the last RGE_(kin) value, asshown at point 1012 where RGE_(st) is equal to RGE_(kin), and both ofwhich are approximately equal to −22%. Then as RGE accumulates data, itconverges to actual road gradient of approximately −20% at time t₃ asreferenced by numeral 1014.

After the vehicle stops, there is noise present on the longitudinalacceleration signal (a_(x) ^(s)) due to vehicle body oscillations, asreferenced by numeral 1016. The vehicle system filters the longitudinalacceleration signal (a_(x) ^(s)) using a low pass filter having avariable bandwidth, as referenced by the flt waveform at 1018. Thevehicle system computes a filtered longitudinal acceleration value bydynamically filtering the a_(x) ^(s) data using a time constant selectedbased on first and higher order derivatives of a_(x) ^(s). In oneembodiment, the vehicle system compares a longitudinal jerk ({dot over(a)}_(x) ^(s)), or a first order derivative of the longitudinalacceleration, to a threshold jerk. If the jerk ({dot over (a)}_(x)^(s)), is greater than the threshold jerk, then a time constantcorresponding to a lower bandwidth is selected for the low pass filter.Conversely, if the jerk is less than the threshold jerk, a time constantcorresponding to a higher bandwidth is selected for the low pass filter.For example, the threshold jerk may be either the converging threshold820 or the stabilizing threshold 828 described above with reference toFIG. 8. In the illustrated embodiment, the fit waveform corresponds tothe bandwidth, where flt equals 1 when a low bandwidth is applied andflt equals 0 when a high bandwidth is selected.

In another embodiment, a bandwidth is selected from predetermined data(e.g., a lookup table) based on a second order derivative of thelongitudinal acceleration ä_(x) ^(s). By filtering a_(x) ^(s), the roadgradient estimation (RGE_(st)) converges relatively quickly. Forexample, RGE_(st) converges at time t₃, as referenced by numeral 1014,and the time delay between t₂ and t₃ is approximately 1 s.

Referring back to FIG. 7, existing systems for evaluating road gradientmay wait for the longitudinal acceleration signal to converge beforemaking a road gradient estimate. Thus such a system would waitapproximately 3 seconds before estimating road gradient for the vehicleconditions illustrated in FIG. 7. However, as shown in FIG. 10A-10C, thevehicle system reduces this time delay by using an initial value forRGEst that is based on other estimations, and by dynamically filteringthe longitudinal acceleration.

As such the vehicle system provides advantages over existing systems byestimating static road gradient using kinematic equations and dynamicfiltering of the longitudinal acceleration. The vehicle system also setsan initial value of the static road gradient estimation equal to a roadgradient provided by a kinematic or dynamic road gradient estimation. Bydynamically filtering the longitudinal acceleration, and by usinganother estimation for the initial value, the vehicle system reduces thetime delay before a quality static road gradient estimated value may beprovided, which in turn allows for the engine to be shutdown earlier ina start/stop application which results in improved fuel economy.

With reference to FIG. 11, a schematic block diagram illustrating amethod for kinematic road gradient estimation is illustrated inaccordance with one or more embodiments and is generally referenced bynumeral 1110. The method 1110 corresponds to the kinematic road gradientestimation portion of the overall RGE arbitration method 410 illustratedin FIG. 4. Block 1112 corresponds to block 412 of FIG. 4. At block 1112the vehicle system receives inputs including acceleration (a_(x) ^(s),a_(y) ^(s), a_(z) ^(s)), yaw rate (r), and vehicle speed (Vx). In oneembodiment, the vehicle system only receives the longitudinalacceleration (a_(x) ^(s)) and vehicle speed. In other embodiments, thevehicle system also receives GPS information (e.g., vehicle speed,acceleration, and elevation). The input may be received directly fromsensors described with respect to FIG. 1, or indirectly over the CANbus.

Blocks 1114 and 1116 correspond to the RGE_(kin) block 424 of FIG. 4. Atblock 1114, the vehicle system determines extended Kalman Filter (EKF)parameters Y_(k) and u_(k), which represent a system output and inputrespectively. The input parameters are based on vehicle speed (V_(x)),longitudinal acceleration (a_(x) ^(s)), and an acceleration offset a_(x)^(off). The parameters may also be based on optional state informationfor embodiments using GPS information. At block 1116, the vehicle systemestimates RGE_(kin) using the EKF and determines the kinematic qualityfactor (QF_(kin)).

The (a_(x) ^(s), a_(z) ^(s) and r) inputs are provided to an RGE_(kin)input compensation block 1118, which corresponds to block 430 of FIG. 4.At block 1118, the vehicle system determines the acceleration offseta_(x) ^(off). The acceleration offset is provided as a feed-forwardinput compensation value for the kinetic road gradient estimation ofblocks 114 and 116. The determination of the acceleration offset isdescribed in detail with respect to FIGS. 14-17.

The vehicle system estimates a kinematic road gradient based on thekinematic vehicle frame equations 1-4 and uses an integral typefiltering strategy. By ignoring high order angular terms, the vehicleframe kinematic equations may be simplified as shown in equations 7-9below:

$\begin{matrix}{{a_{x}^{2} + {l_{c}\overset{.}{r}} - \left( {{\overset{.}{V}}_{x} - {r*V_{y}}} \right)} = {{{- \left( {{\overset{.}{V}}_{x} - {r*V_{y}}} \right)}\frac{\theta^{2}}{2}} + {g\left( {\alpha_{r} + \theta} \right)}}} & (7) \\{{a_{y}^{2} + {l_{d}\overset{.}{r}} - \left( {{\overset{.}{V}}_{y} + {r*V_{x}}} \right)} = {{{- \left( {{\overset{.}{V}}_{y} + {r*V_{x}}} \right)}\frac{\phi^{2}}{2}} - {g\left( {\phi_{v} + \phi_{r}} \right)}}} & (8) \\{{a_{z}^{s} - g} = {{{\overset{.}{V}}_{x}\theta} - {\frac{g}{2}\left( {\alpha_{r} + \theta} \right)^{2}}}} & (9)\end{matrix}$

An equation for longitudinal acceleration ({dot over (V)}_(x)) may beobtained by applying small angle approximation to equation 8, and isshown below:

$\begin{matrix}\begin{matrix}{{\overset{.}{V}}_{x} = a_{x}} \\{= {a_{x}^{s} + {l_{c}\overset{.}{r}} + {r*V_{y}} - {g\;\theta} - {g\;\alpha_{r}}}} \\{\approx {a_{x}^{s} + {l_{c}\overset{.}{r}} + {r*V_{y}} - {g\;\sin\;\theta} - {g\;\sin\;\alpha_{r}}}} \\{= {a_{x}^{s} + a_{x}^{off} - {g\;\sin\;\alpha_{r}}}}\end{matrix} & (10)\end{matrix}$where a_(x) ^(off) represents an acceleration offset.

The vehicle system estimates kinematic road gradient using a Kalmanfilter, according to one or more embodiments. The Kalman filter operatesrecursively on streams of noisy input data to provide a statisticallyoptimal estimate of the underlying system state. Basically, the Kalmanfilter is an algorithm which uses a series of measurements that areobserved over time and contain random variations (noise), and providesestimates of unknown variables that are more accurate than themeasurements alone. A Kalman filter may be thought of as an approach toestimate the state of a “black box”, based on measurable inputs andoutputs.

The Kalman filter algorithm estimates a state using a two-step processincluding a time update prediction step, and a measurement update step.In the prediction step, the Kalman filter provides estimates of thecurrent state variables, along with their uncertainties. The measurementupdate step is a form of feedback control. Once the Kalman filterobserves the next measurement, the estimates are updated using aweighted average, where more weight is given to estimates with highercertainty.

The weights are calculated from the covariance, which is a measure ofthe estimated uncertainty of the prediction of the system's state. TheKalman filter process is repeated every time step, where the newestimate and its covariance informs the prediction used in the followingiteration. The Kalman filter is a recursive estimator. This means thatonly the estimated state from the previous time step and the currentmeasurement are used to compute the estimate for the current state.

The Kalman Filter provides estimates for linear system models withadditive independent white noise in both the transition and themeasurement systems. However, most systems are nonlinear. An extendedKalman filter (EKF) is used to provide estimates of nonlinear systems.The EKF adapts techniques including multivariate Taylor Seriesexpansions, to linearize calculations about a working point.

The vehicle system defines an extended state (a_(x) ^(g)). Since gravityis a constant value (9.8 m/s²), a first order dynamic of the extendedstate is equal to zero ({dot over (a)}_(x) ^(g)=0). The extended statemodels the vehicle acceleration dynamic that is excited by variation inthe road gradient, and the road gradient is relatively high at lowfrequencies.

The overall system model for estimation is shown below, and isreferenced as equation 11:

$\begin{matrix}{\mspace{85mu}{{X_{k} = {{{f\left( {X_{k - 1},u_{k}} \right)} + {\omega_{k}\begin{bmatrix}{V_{x}(k)} \\{a_{x}^{g}(k)}\end{bmatrix}}} = {{\begin{bmatrix}{{V_{x}\left( {k - 1} \right)} - {{a_{x}^{g}\left( {k - 1} \right)}\tau_{s}}} \\{a_{x}^{g}\left( {k - 1} \right)}\end{bmatrix} + {\left\lbrack \begin{matrix}1 \\0\end{matrix} \right\rbrack\left( {{a_{x}^{s}(k)} + {a_{x}^{off}(k)}} \right)\tau_{s}} + {\begin{bmatrix}{\omega_{v}(k)} \\{\omega_{a}(k)}\end{bmatrix}\mspace{20mu} X_{k}}} = {{\begin{bmatrix}{V_{x}(k)} & {a_{x}^{g}(k)}\end{bmatrix}^{T}\mspace{20mu} u_{k}} = {\left( {{a_{x}^{s}(k)} + {a_{x}^{off}(k)}} \right)\tau_{s}}}}}}\mspace{20mu}{Y_{k} = {{\begin{bmatrix}1 & 0\end{bmatrix}X_{k}} = V_{x}}}\mspace{79mu}{\omega_{k} = \begin{bmatrix}{\omega_{v}(k)} & {\omega_{a}(k)}\end{bmatrix}^{T}}}} & (11)\end{matrix}$where X_(k) is the state at time step (k), and X_(k−1) is the state at aprevious time step. u_(k) is a system input and Y_(k) is a systemoutput. ω_(k) is the process noise, ω_(v)(k) is the process noiseassociated with the velocity, and ω_(a)(k) is the process noiseassociated with acceleration. V_(x) is the longitudinal speed. a_(x)^(g)(k) is the extended state acceleration due to gravity, a_(x) ^(s)(k)is the measured longitudinal acceleration, and a_(x) ^(off)(k) is theacceleration offset. τ_(s) is the sampling time for the above discretetime implementation model, and T indicates a matrix transform.

Based on the system model shown in equations 11, an observer is designedto estimate the extended state (a_(x) ^(g)), and the kinematic roadgradient is estimated according to equation 12 as shown below:

$\begin{matrix}{{\hat{\alpha}}_{r} = {{\arcsin\left( \frac{a_{x}^{g}}{g} \right)} = {RGE}_{kin}}} & (12)\end{matrix}$

The observer is designed using a standard Extended Kalman Filter (EKF)system model as represented by the function shown above in equation 11,and equation 13, both of which are shown below:X _(k) =f(X _(k−1) ,u _(k))+ω_(k)Y _(k) =CX _(k) +e _(k)  (13)Where C is a matrix, which is a relatively simple matrix in equation 11,and e_(k) is an unknown noise.

An EKF is used for nonlinear systems. The nonlinear system model islinearized around the current state at every time step, and provides atransition matrix F_(k). The obtained transition matrix F_(k) is thenused to complete the steps of the standard EKF recursions. Theserecursions are described by two update steps: a prediction step and ameasurement update step. In the prediction step, the system input isused to predict the future state of the system ({circumflex over(X)}_(k|k−1)) as shown in equations 14 and 15 below:{circumflex over (X)} _(k|k−1) =f({circumflex over (X)} _(k−1|k−1) ,u_(k))  (14)P _(k|k−1) =F _(k) P _(k−1|k−1) F _(k) ^(T) +Q _(k)  (15)where P_(k|k−1) is the estimated error covariance and Q_(k) is theprocess noise covariance which is equal to E[ω_(k) ²]. F_(k) is theJacobian and is provided by equation 16 as shown below:

$\begin{matrix}{F_{k} = {\frac{\partial f}{\partial x}❘{\hat{x}}_{{k - 1}❘{k - 1}}}} & (16)\end{matrix}$

A measurement update step is provided by equations 17-19 as shown below:K _(k) =P _(k|k−1) C ^(T)(CP _(k|k−1) C ^(T) +R _(k))⁻¹  (17){circumflex over (X)} _(k|k) ={circumflex over (X)} _(k|k−1) +K _(k)(Y_(k) −C{circumflex over (X)} _(k|k−1))  (18)P _(k|k)=(I−K _(k) C)P _(k|k−1)  (19)where K_(k) is the EKF gain, and R_(k) is the measurement noisecovariance which is equal to E[e_(k) ²].

Generally with an EKF system, the process noise covariance (Q) and themeasurement noise covariance (R) are not known a priori, or independentof experience. However, well conditioned signals, such as K_(k) andP_(k)|_(k) will converge to values that are dependent on Q and R.Therefore, the converged value of K_(k) may be obtained frompredetermined data and it will be used directly for real-timeestimation.

In one or more embodiments, the vehicle system includes verticaldisplacement in its estimation of kinematic road grade. Verticaldisplacement (Z) may be provided by the navigation system of FIG. 1based on GPS information. The governing vehicle vertical kineticequation in the global inertia frame is calculated according to equation20 as shown below:Ż=V _(x) sin α_(r)  (20)

The EKF system model of equation (11) is modified to include verticaldisplacement Z(k), as shown in equation 21 below:

$\begin{matrix}{\begin{bmatrix}{V_{x}(k)} \\{Z(k)} \\{a_{x}^{g}(k)} \\{a_{x}^{\theta}(k)}\end{bmatrix} = {{\begin{bmatrix}{{V_{x}\left( {k - 1} \right)} - {{a_{x}^{g}\left( {k - 1} \right)}T_{s}} - {{a_{x}^{\theta}\left( {k - 1} \right)}\tau_{s}}} \\{{Z\left( {k - 1} \right)} + {{V_{x}\left( {K - 1} \right)}\frac{1}{g}{a_{x}^{g}\left( {k - 1} \right)}}} \\{a_{x}^{g}\left( {k - 1} \right)} \\{a_{x}^{\theta}\left( {k - 1} \right)}\end{bmatrix} + {\left\lbrack \begin{matrix}1 \\0 \\0 \\0\end{matrix} \right\rbrack\left( {{a_{x}^{s}(k)} + {a_{x}^{cmp}(k)}} \right)\tau_{s}} + {\begin{bmatrix}{\omega_{v}(k)} \\{\omega_{z}(k)} \\{\omega_{\propto}(k)} \\{\omega_{\theta}(k)}\end{bmatrix}\mspace{20mu} X_{k}}} = {{\begin{bmatrix}{V_{x}(k)} & {Z(k)} & {a_{x}^{g}(k)} & {a_{x}^{\theta}(k)}\end{bmatrix}^{T}\mspace{20mu} u_{k}} = {{\left( {{a_{x}^{s}(k)} + {a_{x}^{cmp}(k)}} \right)T_{s}\mspace{20mu} Y_{k}} = {{CX}_{k} = {{\begin{bmatrix}1 & 0 & 0 & 0 \\0 & 1 & 0 & 0\end{bmatrix}X_{k}} = \begin{bmatrix}{V_{k}(k)} & {Z(k)}\end{bmatrix}^{T}}}}}}} & (21)\end{matrix}$where a_(x) ^(cmp) represents an GPS based acceleration offset, anda_(x) ^(cmp) is equal to (l_(c){dot over (r)}+r*V_(y)). a_(x) ^(cmp) isprovided as feedforward input compensation in embodiments using the GPSinformation. Equation 21 differs from equation 11, in that equation 21includes two extended states: a_(x) ^(g)(k) and a_(x) ^(θ)(k), wherea_(x) ^(θ)(k) is the acceleration due to the vehicle pitch angle.

With reference to FIG. 12, a method for estimating kinematic roadgradient is illustrated in accordance with one or more embodiments andis generally referenced by numeral 1210. The method is implemented usingsoftware code contained within the VSC, according to one or moreembodiments. In other embodiments, the method 1210 is implemented inother vehicle controllers, or distributed amongst multiple vehiclecontrollers.

At operation 1212 the vehicle system initializes estimator parametervalues. The estimator parameter values include X_(k), u_(k), and Y_(k).The estimator parameter values are initialized by setting them equal todefault values. At operation 1214 the vehicle system determines if thekinematic estimator status (STATUS_(kin)) is on. The estimation statusfactor is defined as a Boolean variable (TRUE/FALSE), where TRUEindicates that the status is on, and FALSE indicates that the status isnot on. A status value of TRUE indicates that the estimation method isactively monitoring the vehicle dynamic states (e.g., Vx, a_(x) ^(s),a_(y) ^(s), a_(z) ^(s), and r) and the road gradient information and itsestimation channel are updating. If the determination at operation 1214is positive, the vehicle system proceeds to operation 1216.

At operation 1216 the vehicle system evaluates the present vehicle speed(Vx) to determine if Vx is greater than or equal to a threshold vehicleentry speed (Vspd_(entry)). In one embodiment, the threshold vehicleentry speed is approximately 7 kph (4.3 mph). If the determination atoperation 1216 is negative, the vehicle system proceeds to operation1218. At operation 1218 the vehicle system evaluates Vx to determine ifit is less than a threshold vehicle exit speed (Vspd_(exit)). In oneembodiment, the threshold vehicle exit speed is approximately 3 kph (1.9mph). If the determination at operation 1218 is positive, then thevehicle system proceeds to operation 1220.

At operation 1220, the vehicle system sets the kinematic estimationstatus (STATUS_(kin)) to pause. By setting STATUS_(kin) to pause, thevehicle system stops the estimation and retains the current RGEkinvalue, rather than resetting the value to a default value. At operation1222 the vehicle system evaluates the present speed (Vx) to determine ifVx is less than a threshold vehicle standstill speed (Vspd_(stand)). Thevehicle standstill speed corresponds to the very low vehicle speedreferenced in column 6 of Table A. In one embodiment, the thresholdvehicle standstill speed is approximately 1 kph (0.6 mph). If thedetermination at operation 1222 is positive, the vehicle system proceedsto operation 1224 and sets the kinematic estimation status(STATUS_(kin)) to off (FALSE). If the determination at operation 1216 ispositive, or the determination at operation 1218 is negative, then thevehicle system proceeds to operation 1226.

Operations 1226-1236 relate to the extended Kalman filter operations,which are described above with reference to equations 11-21. First, atoperation 1226 the vehicle system computes a compensated estimator input(u_(k)). At operation 1228 the vehicle system obtains a new systemoutput (Y_(k)). At operation 1230 the vehicle system updates the EKFstate (X_(k)) based on output prediction error, as described above withreference to equations 14-16. At operation 1232 the vehicle systemupdates the EKF parameter values (X_(k), u_(k), and Y_(k)). At operation1234, the vehicle system predicts the next system dynamic output Y_(k).Then at operation 1236 the vehicle system provides the kinematic roadgradient estimation.

If the determination at operation 1214 is negative, the vehicle systemproceeds to operation 1238. At operation 1238 the vehicle systemevaluates the present vehicle speed (Vx) to determine if Vx is greaterthan or equal to the threshold vehicle entry speed (Vspd_(entry)), whichis similar to the determination at operation 1216. In one embodiment,the threshold vehicle entry speed is approximately 7 kph (4.3 mph). Ifthe determination at operation 1238 is positive, the vehicle systemproceeds to operation 1240 and sets the kinematic estimation status(STATUS_(kin)) to on (TRUE). After operation 1240 the vehicle systemproceeds to operation 1226 to begin the EKF operations.

After operations 1224 and 1244, and after negative determinations atoperations 1222 and 1238, the vehicle system proceeds to operation 1242.After operation 1242 the vehicle system returns to operation 1214 foranother iteration.

FIGS. 13A-13D are graphs illustrating the impact of the method 1210 forestimating kinematic road gradient when a vehicle stops on an incline.FIGS. 13A-13D include 4 graphs of measurement data taken over a commonperiod of time and are generally referenced by numeral 1310. FIG. 13Aillustrates brake pressure (P_(brk)) which is measured within the mastercylinder according to one embodiment. In other embodiments, P_(brk) ismeasured within the brake lines or at the wheel assemblies. FIG. 13Billustrates vehicle speed (Vx). FIG. 13C illustrates the gear selection(PRNDL), where a value of 3 corresponds to the drive (D) gear selection.FIG. 13C also illustrates the state of the brake pedal (S_(bp)), where apositive value indicates that the brake is applied, and a value of zeroindicates that the brake is released. FIG. 13D illustrates the measuredlongitudinal acceleration value (a_(x) ^(s)), along with the static roadgrade estimation (RGE_(st)) and the kinematic road grade estimation(RGE_(kin)).

At time t₁, the vehicle is accelerating, as indicated by the increasingslope of Vx. At time t₂ the vehicle starts decelerating as indicated bythe decreasing slope of Vx. At time t₃, the vehicle stops. At time t₄the vehicle begins accelerating again, and at time t₅ the vehicle beginsdecelerating again. The kinematic road grade estimation (RGEkin)converges relatively quickly, as referenced by numerals 1312 and 1314.

As described with respect to operations 1222 and 1224, the kinematicestimation status (STATUS_(kin)) is set to off, if the vehicle speeddecreases below a vehicle standstill threshold speed (e.g., 1 kph). Suchconditions are illustrated by RGE_(kin) and referenced by numeral 1320.After t₃, the estimation status is set to off as depicted by the lack ofany perceptible noise on RGE_(kin) at 1320. Additionally once thevehicle stops, the status of the static road gradient estimation(STATUS_(st)) and the RGE_(st) converges as referenced by numeral 1322.As shown at 1320, even though the kinematic status is off, RGE_(kin) isstill locked at an accurate road grade estimate and both RGE_(st) andRGE_(kin) are providing generally equal estimates of 17.56% grade.

Existing methods for estimating road gradient using a kinematic approachuse a derivative filtering strategy. For example, some existing methodsestimate road gradient based on the following vehicle longitudinalkinematic equation:{dot over (u)}=a _(x) +rv−g sin θ  (22)where {dot over (u)} is the derivative of vehicle longitudinal speed,a_(x) is the vehicle longitudinal acceleration, r is the yaw rate, v isthe vehicle lateral speed. However, such an approach amplifies any noiseon the signal due to the derivative calculation of {dot over (u)}.

As such the vehicle system provides advantages over the existing methodsby estimating kinematic road gradient using an EKF, which provides anintegral type filtering strategy. Such an integrating strategy does notamplify noise on the signals, and therefore provides improved estimationaccuracy with relatively fast convergence.

The vehicle system estimates kinematic road gradient using a feedforwardinput compensation corresponding to an acceleration offset (a_(x)^(off)). The acceleration offset is calculated according to equation 23,as shown below:a _(x) ^(off) =l _(c) {dot over (r)}+rV _(y) −g sin θ  (23)where l_(c) is the lateral offset, r is the yaw rate, V_(y) is thelateral velocity and θ is the vehicle pitch angle. In one or moreembodiments, the vehicle system does not receive these values (l_(c),V_(y), and θ) as inputs. Rather the vehicle system estimates thesevalues from other inputs using a number of compensation strategies. Inone or more embodiments, the vehicle system estimates the lateralvelocity (V_(y)) term of equation 23 using a Coriolis Effectcompensation strategy, the lateral offset (l_(c)) term using a sensorlateral position offset effect compensation strategy, and the vehiclepitch angle (θ) using a vehicle body relative pitch effect compensationstrategy.

The vehicle system estimates the rV_(y) term of equation 23 using aCoriolis Effect compensation strategy which relates to vehicle turningmaneuvers. The inertial sensors 52 of FIG. 1, provide the yaw rate,however the lateral velocity (V_(y)) is estimated by the vehicle system.Two computation methods are provided for estimating the lateral velocity(V_(y)): a first method, which is an estimation based method, and asecond method, which is a derivation based method. The two methods areprovided based on the lateral dynamic equation 8 and a vehicle corneringstiffness parameter, respectively. Furthermore, the two methods areintegrated to provide the estimated lateral velocity (V_(y)) undercontinuous varying vehicle driving conditions.

The first method (the estimation based method) for estimating lateralvelocity (V_(y)) is obtained by applying small angle approximation toEquation 8, and is based on the measured lateral acceleration (a_(y)^(s)). Lateral velocity (V_(y)) is calculated according to equation 24as shown below:{dot over (V)} _(y) =a _(y) ^(s) −l _(d) {dot over (r)}−r*V _(x) +gϕ_(v) +gϕ _(r)  (24)

The road bank angle (ϕ_(r)) is unknown. Therefore it is treated as adisturbance and removed from the calculation. The vehicle body rollangle ϕ_(v) may be approximated by the measured lateral acceleration asshown in equation 25:ϕ_(v) ≈K _(ϕ)(a _(y) ^(s) −l _(d) {dot over (r)})  (25)where K_(ϕ) represents the vehicle body roll stiffness.

Equation 24 may be simplified as shown in equation 26 below:{dot over (V)} _(y)=(1+K _(ϕ) g)(a _(y) ^(s) −l _(d) {dot over (r)})−r*V_(x) =K _(ay)(a _(y) ^(s) −l _(d) {dot over (r)})−r*V _(x)  (26)

By integrating both sides of equation 26, an equation is obtained forcalculating the estimated lateral velocity ({circumflex over(V)}_(y)(k), as shown by equation 27:{circumflex over (V)} _(y)(k)={circumflex over (V)} _(y)(k−1)+[K _(ay)(a_(y) ^(s) −l _(a) {dot over (r)})−r*V _(x)]*τ_(s)  (27)

The measured lateral acceleration (a_(y) ^(s)) is generally subjected toacceleration disturbances and bank angle impact. Therefore the V_(y)estimation method described in equations 24-27 is suited for short timeintervals of vehicle dynamic maneuvers in which large magnitudes oflateral acceleration (a_(y) ^(s)) and yaw rate (r) are present, due tothe low frequency biased acceleration components.

Unlike the first method, the second method (the derivation based method)for estimating the lateral velocity (V_(y)) does not depend onintegration. A steady state gain (G_(r2β)) from the vehicle yaw rate (r)to a vehicle side slip angle (β) may be obtained from a linear vehiclemodel as shown in equation 28:

$\begin{matrix}{G_{r\; 2\;\beta} = {\frac{l_{r}}{V_{x}} - {\frac{M_{r}}{C_{\alpha\; r}}V_{x}}}} & (28)\end{matrix}$where M_(r) is the vehicle mass distributed to the rear axle and C_(ar)is a lumped rear axle cornering stiffness.

As the lateral velocity (V_(y)) becomes approximately equal to theproduct of the longitudinal velocity (V) and the vehicle side slip angle(β), an equation for calculating the lateral velocity may be obtained asshown below:V _(y) =V _(x) G _(r2β) r=(l _(r) −k _(U) V _(x) ²)*r=K _(r)(V_(x))*r  (29)where k_(U) is the cornering stiffness and is equal to M_(r) divided byC_(ar).

Therefore the lateral velocity may be estimated by equation 30 as shownbelow:{circumflex over (V)} _(y) =r*(l _(r) −k _(U) V _(x) ²)=K _(r)(V_(x))*r  (30)

A Coriolis effect compensation equation is provided below:

$\begin{matrix}{{r*{\hat{V}}_{y}} = {{r^{2}*\left( {l_{r} - {\frac{M_{r}}{C_{\alpha\; r}}V_{x}^{2}}} \right)} = {r^{2}*\left( {l_{r} - {k_{U}V_{x}^{2}}} \right)}}} & (31)\end{matrix}$

The second method as described with reference to equations 28-31 issuited for steady state vehicle turning maneuvers where the side slipangle (β) is small.

The vehicle system implements a comprehensive RGE_(kin) structure thatincorporates both the first method (estimation based) and the secondmethod (derivation based). Then, the vehicle system selects which of thetwo methods to apply, based on vehicle dynamic conditions reflected bythe yaw rate (r). The general design concept is to use the derivationbased V_(y) from equation 30, in primarily forward driving and steadystate turning situations but to switch to the estimation based V_(y)from equation 27, in the presence of large vehicle lateral dynamics(speed and acceleration). Since the estimation method is not expected tobe carried out for a long period of time, the estimated V_(y) must bereset to the derived state whenever a steady state condition may beobserved during a continuous vehicle handling maneuver. The estimatedV_(y) may be further bounded by predetermined upper and lower limits.The output V_(y) is provided by a low pass filter.

With reference to FIG. 14, a method for determining a vehicle drivingsituation is illustrated in accordance with one or more embodiments andis generally referenced by numeral 1410. The method is implemented usingsoftware code contained within the VSC, according to one or moreembodiments. In other embodiments, the method 1410 is implemented inother vehicle controllers, or distributed amongst multiple vehiclecontrollers.

The method 1410 includes evaluating an indexed lateral dynamic (ILD),which correspond to a present vehicle driving situation. The ILD isdefined as:ILD=|r|+a*|{dot over (r)}  (32)where the variable (a) is a scaling parameter. The vehicle systemcompares the ILD to two thresholds values: a steady state threshold(SST) corresponding to the derivation method, and a dynamic handlingthreshold (DHT) corresponding to the estimation method, where (SST<DHT).The determination is further made together with two timers, a steadystate timer (T_(ss)) and a dynamic driving timer (T_(dd)) that count theduration a vehicle has stayed in each situation. Function LPF(x)indicates a general low pass filter function. A moving average filter isan example of a low pass filter that may be used to smooth outdiscontinuous output transition effects.

In operation 1412 the vehicle system determines if the vehicle is in thesteady state driving situation. The vehicle system receives a flag(SSD_(flag)) that is associated with the driving situation. A SSD_(flag)that is equal to one (1) indicates a present steady state drivingsituation, and SSDn_(flag) that is equal to two (2) indicates a presentdynamic handling situation. Thus, at operation 1412, the systemevaluates the SSD_(flag) to determine if SSD_(flag)=2. If thedetermination at operation 1412 is positive, the vehicle system proceedsto operation 1414.

At operation 1414, the vehicle system evaluates the indexed lateraldynamic (ILD) to determine if the ILD is less than a steady statethreshold (SST). The SST corresponds to the derivation method fordetermining the lateral velocity (V_(y)) as described above withreference to equations 28-31. If the determination at operation 1414 isnegative, the vehicle system proceeds to operation 1416 and resets asteady state timer (T_(ss)) by setting T_(ss) equal to zero (0). If thedetermination at operation 1414 is positive, the vehicle system proceedsto operation 1418.

At operation 1418 the vehicle system increments the steady state timer(T_(ss)) by counting up by one execution cycle time (t_(s)). Atoperation 1420 the vehicle system evaluates the steady state timer(T_(ss)) to determine if T_(ss) is greater than or equal to a settlingtime threshold (tss_(settle)). If the determination at operation 1420 ispositive, the vehicle system proceeds to operation 1422 and resets thedynamic driving timer (T_(dd)). Then, at operation 1424 the vehiclesystem sets the vehicle driving situation to steady state by setting theSSD_(flag) equal to one (1).

If the determination at operation 1412 is negative (the vehicle is notdriving in steady state conditions), the vehicle system proceeds tooperation 1426. At operation 1426 the vehicle system evaluates theindexed lateral dynamic (ILD) to determine if the ILD is greater thanthe dynamic handling threshold (DHT). The DHT corresponds to theestimation method for determining the lateral velocity (V_(y)) asdescribed above with reference to equations 24-27. If the determinationat operation 1426 is negative, the vehicle system proceeds to operation1428 and resets a dynamic driving timer (T_(dd)) by setting T_(dd) equalto zero (0). If the determination at operation 1426 is positive, thevehicle system proceeds to operation 1430.

At operation 1430 the vehicle system increments the dynamic drivingtimer (T_(dd)) by counting up by one execution cycle time (t_(s)). Atoperation 1432 the vehicle system evaluates the dynamic driving timer(T_(dd)) to determine if T_(dd) is greater than or equal to anexcitation time threshold (tdd_(excite)). If the determination atoperation 1432 is positive, the vehicle system proceeds to operation1434 and resets the steady state timer (T_(ss)). Then, at operation 1436the vehicle system sets the vehicle driving situation to dynamic drivingby setting the SSD_(flag) equal to two (2). After operations 1416, 1424,1428 and 1436, and after a negative determination at operations 1420 or1432, the vehicle system proceeds to operation 1438 and then returns tooperation 1412 for another iteration.

With reference to FIG. 15, a method for estimating a vehicle lateralvelocity (V_(y)) based on a present vehicle driving situation isillustrated in accordance with one or more embodiments and is generallyreferenced by numeral 1510. The method is implemented using softwarecode contained within the VSC, according to one or more embodiments. Inother embodiments, the method 1510 is implemented in other vehiclecontrollers, or distributed amongst multiple vehicle controllers. Themethod 1510 arbitrates, by selecting a method for estimating lateralvelocity (V_(y)), from one of the two methods described above withreference to equations 24-31; either the integration estimation basedmethod or the derivation estimation based method

In operation 1512 the vehicle system determines if the vehicle is in thedynamic driving situation. The vehicle system evaluates the SSD_(flag)to determine if SSD_(flag)=2. If the determination at operation 1512 ispositive, the vehicle system proceeds to operation 1514, and estimatesthe lateral velocity ({circumflex over (V)}_(y)) using the integrationestimation provided in equation 27. After operation 1514 the vehiclesystem proceeds to operation 1516. At operation 1516 the vehicle systemfilters the estimated lateral velocity ({circumflex over (V)}_(y)) usinga low pass filter (LPF), and updates a lateral velocity output (V_(y)).If the determination at operation 1512 is negative, the vehicle systemproceeds to operation 1518. At operation 1518 the vehicle systemestimates the lateral velocity ({circumflex over (V)}_(y)) using thederivation estimation provided in equation 30. After operation 1518, thevehicle system proceeds to operation 1516, filters the estimated lateralvelocity ({circumflex over (V)}_(y)) using a low pass filter (LPF), andupdates the lateral velocity output (V_(y)) based on the derivationestimation. After operation 1516, the vehicle system proceeds tooperation 1520 and provides the updated lateral velocity (V_(y)) output,and then returns to operation 1512.

In one or more embodiments, the vehicle system estimates the lateraloffset (l_(c)) term using a sensor lateral position offset effectcompensation strategy. The lateral offset (l_(c)) is defined by thefollowing equation:l _(c) =l _(c) ^(st) +h ^(t)ϕ_(v)  (33)where l_(c) ^(st) is a static sensor installation offset. The variableh^(t)ϕ_(v) is a dynamic vehicle body roll component, where h^(t) is thedistance from the sprung mass C.G. to the roll center. The vehicle rollangle ϕ_(v) may be obtained from a roll dynamic model or from suspensiondeflections (in embodiments having suspension deflection sensors). Inone or more embodiments, ϕ_(v) at steady state is derived from the yawrate (r) at steady state as shown below:ϕ_(v) =G _(r2ϕ) r  (34)

Combining equations 33 and 34 and multiplying each side by thederivative of yaw rate ({dot over (r)}) provides the following equation:l _(c) {dot over (r)}=l _(c) ^(st) {dot over (r)}+h ^(t) G _(r2ϕ)*r*{dot over (r)}  (35)

In a dynamic driving situation, the body roll angle (ϕ_(v)) isapproximated from the lateral acceleration (a_(y) ^(s)) according toequation 36 as shown below:ϕ_(v) =G _(ay2ϕ)(a _(y) ^(s) −l _(d) {dot over (r)})  (36)

Combining equations 35 and 36 provides the following equation:l _(c) {dot over (r)}=(l _(c) ^(st) −h ^(t) G _(ay2ϕ) l _(d)){dot over(r)}+h ^(t) G _(ay2ϕ) a _(y) ^(s)  (37)where G_(ay2ϕ) is equal to K_(phi).

With reference to FIG. 16, a method for estimating the body roll angle(ϕ_(v)) based on a present vehicle driving situation is illustrated inaccordance with one or more embodiments and is generally referenced bynumeral 1610. The method is implemented using software code containedwithin the VSC, according to one or more embodiments. In otherembodiments, the method 1610 is implemented in other vehiclecontrollers, or distributed amongst multiple vehicle controllers. Themethod 1610 arbitrates, by selecting a method for estimating the bodyroll angle (ϕ_(v)), from one of the two methods described above withreference to equations 33-37; either using lateral acceleration or yawrate.

In operation 1612 the vehicle system determines if the vehicle is in thedynamic driving situation. The vehicle system evaluates the SSD_(flag)to determine if SSD_(flag)=2. If the determination at operation 1612 ispositive, the vehicle system proceeds to operation 1614, and estimatesthe body roll angle ({circumflex over (ϕ)}_(v)) based on lateralacceleration as shown by equation 36. After operation 1614 the vehiclesystem proceeds to operation 1616. At operation 1616 the vehicle systemfilters the estimated body roll angle ({circumflex over (ϕ)}_(v)) usinga low pass filter (LPF), and updates a body roll angle output (ϕ_(v)).If the determination at operation 1612 is negative, the vehicle systemproceeds to operation 1618. At operation 1618 the vehicle systemestimates the body roll angle ({circumflex over (ϕ)}_(v)) based on theyaw rate as provided in equation 34. After operation 1618, the vehiclesystem proceeds to operation 1616, filters the estimated body roll angle({circumflex over (ϕ)}_(v)) using a low pass filter (LPF), and updatesthe body roll angle output (ϕ_(v)). After operation 1616, the vehiclesystem proceeds to operation 1620 and provides the updated body rollangle output (ϕ_(v)), and then returns to operation 1612.

In one or more embodiments, the vehicle system estimates the vehiclebody pitch angle (θ) using one of three compensation strategies. Thedifferent strategies correspond to the availability of sensors andaccuracy. For example, a first strategy estimates the pitch angle (θ)based on the vehicle longitudinal speed (V_(x)) and the vehiclelongitudinal acceleration (a_(x) ^(s)). A second strategy, estimates thepitch angle (θ) based on V_(x), a_(x) ^(s), and a vertical accelerationa_(z) ^(s). A third strategy estimates the pitch angle (θ) based onV_(x), a_(x) ^(s), a_(z) ^(s) and an estimated acceleration due togravity component a_(x) ^(g) provided by the extended Kalman filter.Generally, the more input signals used and the more accurate the sourceinformation is, the better the compensation strategy is at cancellingthe coupling effect of the a_(x) ^(g) estimation, and the better theoverall road gradient estimation.

The first compensation strategy for estimating the vehicle body pitchangle (θ), is referenced as dynamic pitch compensation. For thisstrategy, the vehicle system estimates the pitch angle (θ) based on thevehicle longitudinal speed (V_(x)) and the vehicle longitudinalacceleration (a_(x) ^(s)). A low frequency component of θ may beapproximated by a first or second order linear dynamic system. Thethrust to pitch vehicle dynamic is generally caused by the longitudinalforces exerted on the vehicle body according to the following equation:

$\begin{matrix}{{\theta \propto {K_{p}a_{x}^{r}}} = {\frac{{M_{s}\left( {K_{f} + K_{r}} \right)}h_{cg}}{K_{f}K_{r}L^{2}}a_{x}^{r}}} & (38)\end{matrix}$where θ is proportional to K_(p)a_(x) ^(r), and M_(s) is a vehiclesprung mass, which is the vehicle mass and payload above the suspension,and h_(cg) is the height of the vehicle CG above ground in the verticaldirection. K_(f) and K_(r) are predetermined suspension stiffness at thefront axle and the rear axle, respectively. a_(x) ^(r) is an equivalentreactive acceleration and a_(x) ^(r) is equal to a_(x)+g sin α_(r). Aninitial thrust to the pitch dynamic model is based on a_(x) ^(r) andshown below:θ_(raw) =K _(p)({dot over (V)} _(x) +a _(x) ^(g))  (39)

A first order relative pitch dynamic is provided by equation 40 as shownbelow:

$\begin{matrix}{{\theta + {\frac{D_{pitch}}{K_{pitch}}\overset{.}{\theta}}} = \theta_{raw}} & (40)\end{matrix}$which has a transfer function of:

$\begin{matrix}{{T(S)} = \frac{K_{pitch}}{K_{pitch} + {D_{pitch}S}}} & (41)\end{matrix}$where D_(pitch) represents the general damping effect of the suspensionand K_(pitch) represents the suspension stiffness to the pitch thrustfrom the longitudinal force.

FIG. 17 depicts a diagram for selecting D_(pitch) and K_(pitch) using aphase plane gain scheduling rule, which is generally represented bynumeral 1710. The diagram 1710 illustrates eight regions, which arecentered about an origin of a Cartesian coordinate system. The x-axiscorresponds to {dot over (V)}_(x)+a_(x) ^(g) and the y-axis correspondsto the pitch angle (θ). The pitch angle θ is bounded by θ_(max) andθ_(min) as shown. Generally, in the normal region, there is a linearrelationship, however in stiffer region there is a non linearrelationship. The values are slightly different between pitching forwardand rearward.

The vehicle system uses the second compensation strategy to estimate thepitch angle (θ) based on V_(x), a_(x) ^(s), and a vertical acceleration(a_(z) ^(s)). The inertial sensors 52 of FIG. 1, provide the verticalacceleration (a_(z) ^(s)) according to one or more embodiments.

For convenience variable (Ψ) may be defined as shown below:Ψ=a _(z) ^(s) +lc{dot over (r)}−({dot over (V)} _(x) −r*V _(y))  (42)

The following equation can be obtained by combining equations 7, 9, and42 and substituting (α_(r)+θ):

$\begin{matrix}{{a_{z}^{s} - g} = {{{\overset{.}{V}}_{x}\theta} - {\frac{1}{2\; g}\left\lbrack {\Psi^{2} + {2\;{\Psi\left( {{\overset{.}{V}}_{x} - {r*V_{y}}} \right)}\frac{\theta^{2}}{2}} + {\left( {{\overset{.}{V}}_{x} - {r*V_{y}}} \right)^{2}\frac{\theta^{4}}{4}}} \right\rbrack}}} & (43)\end{matrix}$

Equation 43 may be simplified by ignoring the high order term with θ⁴,as shown below:Ψ({dot over (V)} _(x) −r*V _(y))θ²−2g{dot over (V)} _(x)θ+Ψ²+2g(a _(z)^(s) −g)=0  (44)

An equation for calculating the relative pitch angle (θ) according tothe second strategy is obtained by solving for the root of equation 44,as shown in equation 45:

$\begin{matrix}{\theta = \frac{{g\;{\overset{.}{V}}_{x}} - \sqrt{{g\;{\overset{.}{V}}_{x}^{2}} - {{\Psi\left( {{\overset{.}{V}}_{x} - {r*V_{y}}} \right)}\left\lbrack {\Psi^{2} + {2\;{g\left( {a_{z}^{s} - g} \right)}}} \right\rbrack}}}{\Psi\left( {V_{x} - {r*V_{y}}} \right)}} & (45)\end{matrix}$

The vehicle system uses the third compensation strategy to estimate thepitch angle (θ) based on V_(x), a_(x) ^(s), and gravity component a_(x)^(g) provided by the extended Kalman filter. An approximation equationis provided below:

$\begin{matrix}{{{\sin\;\theta} \approx \frac{{a_{z}^{s} - {g\;\cos}} \propto_{r}}{a_{x}^{s}}} = \frac{a_{z}^{s} - \sqrt{g^{2} - \left( a_{x}^{g} \right)^{2}}}{a_{x}^{s}}} & (46)\end{matrix}$

The pitch angle (θ) may be iteratively identified together with anestimated road gradient (α_(r)) through regression based on thefollowing equation:

$\begin{matrix}{\quad\left\{ \begin{matrix}{{{\overset{\_}{a}}_{x}^{off}\left( {k + 1} \right)} = {{l_{c}\overset{.}{r}} + {r*V_{y}} - {g\frac{a_{z}^{s} - \sqrt{g^{2} - \left( {{\hat{a}}_{x}^{g}(k)} \right)^{2}}}{a_{x}^{s}}}}} \\{{{\hat{a}}_{x}^{g}\left( {k + 1} \right)} = {{{\overset{\_}{a}}_{x}^{off}\left( {k + 1} \right)} - {{\overset{\_}{a}}_{x}^{off}(k)} + {{\hat{a}}_{x}^{g}(k)}}}\end{matrix} \right.} & (47)\end{matrix}$where â_(x) ^(g)(k=0)=a_(x) ^(g)(t) from the Kalman filter output basedon an initial compensation value of a_(x) ^(off)(t). This value may bezero (0), any other admissible value or the regression result from theprevious cycle. ā_(x) ^(off)(k+=0)=a_(x) ^(off)(t).

Equations for estimating the road gradient (â_(r)) and the relativepitch angle ({circumflex over (θ)}) according to the third compensationstrategy are provided by equations 48 and 49, as shown below:

$\begin{matrix}{{\hat{\alpha}}_{r} = {\arcsin\left( \frac{{\hat{a}}_{x}^{g}(\infty)}{g} \right)}} & (48) \\{\hat{\theta} = {\arcsin\left( \frac{{l_{c}\overset{.}{r}} + {r*V_{y}} - {{\overset{\_}{a}}_{x}^{off}(\infty)}}{g} \right)}} & (49)\end{matrix}$where â_(x) ^(g)(∞) and ā_(x) ^(off)(∞) indicate the converged value ofâ_(x) ^(g)(k) and ā_(x) ^(off)(k) through the regression process as k→∞.The convergence generally occurs within 5-10 iterations.

As such the vehicle system provides advantages over the existing methodsby estimating kinematic road gradient using input compensatedacceleration offset to improve accuracy to remove uncertainties andoffsets caused by lateral vehicle dynamics and their coupling effects.

The vehicle system estimates a dynamic road gradient (RGE_(dyn)) basedon input corresponding to estimated vehicle mass, vehicle speed, wheeltorques (brake torque and powertrain output torque), and other dragforces acting on the vehicle (e.g., aerodynamic drag, rolling resistanceand the road gradient load). Generally the dynamic RGE has lower qualityconditions as compared to static and kinematic RGE because it involvesmore signals in its estimation, and the noise and error in each signalwill be propagated in the final RGE output. However, the dynamic RGEdoes not rely on the longitudinal acceleration signal (a_(x) ^(s)), andtherefore it has different signal coverage from that of the static andthe kinematic estimations. Although dynamic road gradient estimation issecondary, the vehicle mass estimation portion is a useful parameter tomany vehicle systems.

The vehicle system estimates vehicle mass independent of the roadgradient estimate using a recursive least square (RLS) algorithm undercertain vehicle conditions. Generally, the vehicle mass only changeswhen an object (e.g., a passenger) is added to or removed from thevehicle. Such a change usually only occurs when the vehicle is stopped.The vehicle system estimates vehicle mass while the vehicle is movingusing vehicle dynamic equations, and independent of the road gradientestimation. By estimating vehicle mass and road gradient independently,or in parallel, the vehicle system improves the accuracy of eachestimate over existing methods. The vehicle system estimates vehiclemass using an event-seeking strategy, whereby the mass is estimated atvehicle conditions where the signal to noise ratio of certain inputs ishigh. The vehicle system then estimates vehicle mass using a recursiveleast square (RLS) algorithm when the sought event occurs. The vehiclesystem re-initializes certain parameters in the mass estimationalgorithm at vehicle conditions corresponding to potential mass changes(e.g., when the vehicle stops) in order to increase the sensitivity ofthe algorithm.

The vehicle system estimates a vehicle mass using dynamic vehicle frameequations that are based on Newton's Second Law (ΣF=ma) as providedbelow:M _(e) {dot over (V)} _(x) =F _(pwt) −F _(brk) −F _(drag) −F _(g)  (50)where M_(e) is the estimated vehicle mass without payload (passengers orcargo), {dot over (V)}_(x) is the derivative of vehicle speed, F_(pwt)is the propulsion force at the output of the powertrain, F_(brk) is thebrake force, F_(drag) is the drag force and F_(g) is the force due togravity.

Equation 50 may be rewritten to solve for M_(e) and further separatedinto equations based on whether or not the vehicle is in gear, as shownbelow:

$\begin{matrix}{M_{e} = \left\{ \begin{matrix}{M_{v} + \frac{J_{w}}{R_{w}^{2}} + \frac{r_{p}^{2}\eta_{p}^{2}J_{p}}{R_{w}^{2}}} & {{In}\mspace{14mu}{gear}} \\{M_{v} + \frac{J_{w}}{R_{w}^{2}}} & {{Not}\mspace{14mu}{in}\mspace{14mu}{gear}}\end{matrix} \right.} & (51)\end{matrix}$where M_(v) is estimated vehicle mass, which includes the vehicle masswithout payload (M_(e)) and the vehicle payloads. The term “in gear”generally references whether or not the powertrain is connected to thewheels. J_(w) and J_(p) are the moment of inertia at the wheels and thepowertrain, respectively. R_(w) is the wheel radius. r_(p) is the lumpedpowertrain torque transfer ratio, and η_(p) is the lumped torquetransfer efficiency. J_(p), r_(p), and η_(p) vary depending ontransmission gear ratio. These values also vary for hybrid electricvehicles having multiple propulsion devices (e.g., a motor and anengine).

An inertia dependent vehicle mass (M_(i)) is calculated according toequation 52, as shown below:

$\begin{matrix}{M_{i} = \left\{ \begin{matrix}\frac{J_{w} + {r_{p}^{2}\eta_{p}^{2}J_{p}}}{R_{w}^{2}} & {{In}\mspace{14mu}{gear}} \\\frac{J_{w}}{R_{w}^{2}} & {{Not}\mspace{14mu}{in}\mspace{14mu}{gear}}\end{matrix} \right.} & (52)\end{matrix}$

An equivalent longitudinal tire force (F_(pwt)) acting on the wheels maybe calculated based on the total powertrain propulsion torque (T_(pwt)_(_) _(whl)) or the total regenerative braking torque (T_(brk) _(_)_(whl)) according to equation 53 as shown below:

$\begin{matrix}{F_{pwt} = {\frac{T_{pwt\_ whl}}{R_{w}} = {{\frac{1}{R_{w}}\left( {T_{eng\_ whl} + T_{mot\_ whl}} \right)} = {{\frac{r_{pe}\eta_{pe}}{R_{w}}T_{eng}} + {\frac{r_{pm}\eta_{pm}}{R_{w}}T_{mot}}}}}} & (53)\end{matrix}$where T_(eng) and T_(mot) are the engine and electric motor outputtorque, and T_(eng) _(_) _(whl) and T_(mot) _(_) _(whl) are theircorresponding torques at the wheels. r_(pe), r_(pm), η_(pe), and η_(pm)are lumped torque transfer ratio and efficiency parameters of theengine, and the motor respectively.

An equivalent longitudinal brake force (F_(brk)) acting on the wheelsmay be calculated based on the total frictional braking torque accordingto equation 54 as shown below:

$\begin{matrix}{F_{brk} = {\frac{1}{R_{w}}K_{brk}P_{brk}}} & (54)\end{matrix}$where P_(brk) is the effective brake line pressure in the wheel brakechambers at the wheel assemblies. This is an equivalent term evaluatedon all wheels, because individual wheel brake chambers may havedifferent applied brake line pressure. K_(brk) is the frictional brakeeffectiveness factor that determines how much frictional brake torquecan be realized over the braking pads given a certain brake pressure.The value of K_(brk) varies with respect to brake system conditions suchas temperature, humidity, etc. In one embodiment, K_(brk) corresponds toa predetermined constant.

A drag force (F_(drag)) may be calculated according to equation 55 asshown below:

$\begin{matrix}{F_{drag} = {{\frac{1}{2}K\;\rho_{a}A_{d}C_{d}V_{x}^{2}} + {M_{v}g\; f_{r}\cos\;\alpha_{r}}}} & (55)\end{matrix}$where ρ_(a) and C_(d) are constant aerodynamic parameters, A_(d) is avehicle cross sectional area, and f_(r) is the rolling resistancecoefficient.

A load on the vehicle due to gravity (F_(g)) may be calculated accordingto equation 56 as shown below:F _(g) =M _(v) g sin α_(r)  (56)where α_(r) is the road gradient.

Equation 56 may be converted to the torque domain as shown in equation57 below:T _(g) =M _(v) gR _(w) sin α_(r)  (57)where R_(w) is the wheel radius.

A first method is provided for estimating vehicle mass concurrently withroad gradient using kinematic equations using a least-square type ofestimator based on the following parameterized model:

$\quad\begin{matrix}\begin{matrix}{y = {\overset{.}{V}}_{x}} \\{= {\frac{F_{pwt} - F_{brk} - F_{drag}}{M_{e}} - {\frac{M_{v}}{M_{e}}g\;\sin\;\alpha_{r}}}} \\{= {{\left( {F_{pwt} - F_{brk} - F_{drag}} \right)\frac{1}{M_{v} + M_{i}}} - {g\;\frac{M_{v}\sin\;\alpha_{r}}{M_{v} + M_{i}}}}} \\{= {\phi^{T}\gamma}}\end{matrix} & (58)\end{matrix}$

The estimation results for vehicle mass (Mv) and road gradient (α_(r))based on the model of equation 58 are obtained as follows:

$\begin{matrix}{M_{v} = {\frac{1}{\gamma_{1}} - M_{i}}} & (59) \\{\alpha_{r} = {\arcsin\left( \frac{\gamma_{2}}{1 - {\gamma_{1}M_{i}}} \right)}} & (60)\end{matrix}$

However solving for two unknowns (α_(r) and M_(v)) simultaneously addsadditional error to the estimates.

A second approach for estimating the vehicle mass using kinematicequations is to separate the vehicle mass estimate from the roadgradient estimate based on the following mathematical model:

$\begin{matrix}{{a_{x} + {g\;\sin\;\alpha_{r}}} = {{\frac{1}{M_{e}}\left( {F_{pwt} - F_{brk} - F_{drag}} \right)} + {\frac{M_{e} - M_{v}}{M_{e}}g\;\sin\;\alpha_{r}}}} & (61)\end{matrix}$

Equation may be simplified as follows, as described above with referenceto equation 10, and reproduced below:{dot over (V)} _(x) =a _(x) =a _(x) ^(s) +a _(x) ^(off) −g sinα_(r)  (10)where a_(x) ^(off) represents an acceleration offset and is equal to(l_(c){dot over (r)}+r*V_(y)−g sin θ). l_(c) is the lateral offset, r isthe yaw rate, V_(y) is the lateral velocity and θ is the vehicle pitchangle. In one or more embodiments, the vehicle system does not receivethese values (l_(c), V_(y), and θ) as inputs. Rather the vehicle systemestimates these values from other inputs using a number of compensationstrategies.

The following equation is obtained by combining equations 61 and 10:

$\begin{matrix}{{a_{x}^{s} + a_{x}^{off}} = {{\frac{1}{M_{e}}\left( {F_{pwt} - F_{brk} - F_{drag}} \right)} + a_{\delta\; g}}} & (62)\end{matrix}$

The variable (a_(δg)) represents the acceleration disturbance excitedfrom road gradient variation, and may be calculated according toequation 63 as shown below:

$\begin{matrix}{a_{\delta\; g} = {\frac{M_{e} - M_{v}}{M_{e}}g\;\sin\;\alpha_{r}}} & (63)\end{matrix}$where M_(v) is the sum of the vehicle mass M_(e) and vehicle payloads.

The acceleration disturbance (a_(δg)) contributes to the estimationerror. When the vehicle payload is small, the a_(δg) may be ignored.a_(δg) may also be regressively compensated after an estimated vehiclemass is in steady state and converged. The acceleration offset (a_(x)^(off)) may be determined and used as feedforward input compensation asdescribed above with reference to FIGS. 14-17.

Equation 62 may be rearranged to solve for M_(e) and M_(v) as shownbelow:

$\begin{matrix}{M_{e} = {\frac{F_{pwt} - F_{brk} - F_{drag}}{a_{x}^{s} + a_{x}^{off} - a_{\delta\; g}} = \frac{\frac{T_{pwt\_ whl} - {K_{brk}P_{brk}}}{R_{w}} - F_{drag}}{a_{x}^{s} + a_{x}^{off} - a_{\delta\; g}}}} & (64) \\{M_{v} = {\frac{\frac{T_{pwt\_ whl} - {K_{brk}P_{brk}}}{R_{w}} - F_{drag}}{a_{x}^{s} + a_{x}^{off} - a_{\delta\; g}} - M_{i}}} & (65)\end{matrix}$

The drag force (Fdrag) and the road gradient (α_(r)) are compensated inorder to obtain vehicle mass estimates. However, it is difficult toaccurately compensate these terms due to signal noise. These termsexhibit slow dynamic variations as compared to the variations exhibitedby the vehicle longitudinal dynamic. Therefore these terms may betreated as unknown constants in a short period estimation event. Adifference based estimation equation may be derived as follows:a _(x) ^(i)(t)M _(e) =F _(whl)(t)−F _(drag)(t)+M _(e) a _(δg)(t)  (66)a _(x) ^(i)(t+Δt)M _(e) =F _(whl)(t+Δt)−F _(drag)(t+Δt)+M _(e) a_(δg)(t+Δt)  (67)where the longitudinal acceleration (a_(x) ^(i)) at time index (i) isequal to (a_(x) ^(s)+a_(x) ^(off)) and the force at the wheels (F_(whl))is equal to (T_(pwt) _(_) _(whl)−K_(brk)P_(brk))/R_(w). The variable (t)is a time instance within a qualified estimation event. At is apredefined time interval parameter.

As F_(drag)(t)≈F_(drag)(t+Δt) and a_(δg)(t)≈a_(δg)(t+Δt), an incrementalvehicle longitudinal dynamic model may be obtained by taking thedifference between equations 66 and 67 as shown below:

$\begin{matrix}{M_{v} = {\left. \frac{\Delta\; F_{whl}}{\Delta\; a_{x}^{i}} \middle| {}_{\Delta\; t}{- M_{i}} \right. = {\frac{{F_{whl}\left( {t + {\Delta\; t}} \right)} - {F_{whl}(t)}}{{a_{x}^{i}\left( {t + {\Delta\; t}} \right)} - {a_{x}^{i}(t)}} - M_{i}}}} & (68)\end{matrix}$where ΔF_(whl) and Δa_(x) ^(i) are the difference between the values ofF_(whl) and a_(x) ^(i) evaluated in the time interval Δt within aqualified estimation event.

Estimating vehicle mass (M_(v)) using such an approach isolates thevehicle mass estimate from noise factors such as: aerodynamic noise,rolling resistance and road gradient. As Δt→0, the equation 68 may besimplified to a derivative format, according to equation 69 as shownbelow:

$\begin{matrix}{M_{v} = {{\frac{{\overset{.}{F}}_{whl}}{{\overset{.}{a}}_{x}^{i}} - M_{i}} = {{\frac{1}{R_{w}}\frac{\left( {T_{pwt\_ whl} - {K_{brk}P_{brk}}} \right)^{\prime}}{\left( {a_{x}^{s} + a_{x}^{off}} \right)^{\prime}}} - M_{i}}}} & (69)\end{matrix}$

With reference to FIG. 18, a schematic block diagram illustrating amethod for vehicle mass estimation is illustrated in accordance with oneor more embodiments and is generally referenced by numeral 1810. Themethod 1110 is included within the dynamic road gradient estimation(RGE_(dyn)) block 426 of FIG. 4, according to one or more embodiments.At block 1812 the vehicle system receives inputs including acceleration(a) yaw rate (r), vehicle speed (V_(x)), brake pressure (P_(brk))powertrain torque (T_(pwt)) and braking torque (T_(brk)). The input maybe received directly from sensors described with respect to FIG. 1, orindirectly over the CAN bus.

At block 1814, the vehicle system determines dynamic mass inputparameters Y_(k) and ϕ_(k) which represent a system output, and a systeminput respectively. The input parameters are based on the calculatedvehicle speed (V_(x)) the longitudinal acceleration (a_(x) ^(s)), apowertrain force (F_(pwt)), a braking force (F_(brk)), and anacceleration offset (a_(x) ^(off)). F_(pwt) and F_(brk) are calculatedbased on T_(pwt) and T_(brk). At block 1816, the vehicle systemestimates the dynamic mass (M_(v)). At block 1818, the vehicle systemdetermines the acceleration offset a_(x) ^(off). The acceleration offsetis provided as feed-forward RGE input compensation. At block 1820 thevehicle system estimates road gradient α_(r).

The vehicle system estimates the dynamic vehicle mass (M_(v)) at block1816 using a recursive least square (RLS) estimation according toequation 65, 68 or 69, according to one or more embodiments. The RLSestimation is based on the following parameterized estimation model:y=ϕ*θ  (70)

A recursive least square estimation (RLS) estimation algorithm isdeveloped as shown below:

$\begin{matrix}{{L(k)} = \frac{{P\left( {k - 1} \right)}{\phi(k)}}{1 + {{\phi^{2}(k)}{P\left( {k - 1} \right)}}}} & (71) \\{{\hat{\theta}(k)} = {{\hat{\theta}\left( {k - 1} \right)} + {{L(k)}\left\lbrack {{y(k)} - {{\phi(k)}*{\hat{\theta}(k)}}} \right\rbrack}}} & (72) \\{{P(k)} = {\left\lbrack {1 - {{L(k)}{\phi(k)}}} \right\rbrack{P\left( {k - 1} \right)}}} & (73)\end{matrix}$where {circumflex over (θ)}(k) is the estimated version of parameter θat k-th execution cycle. The RLS estimator is a filtering strategy thataverages the estimated data updated based on the error between themeasurement and the predicted model output. Such an estimation strategymay be used when the parameter to be estimated is constant in nature. Toinitiate the estimation, P(0) is set equal to 1.

The parameters of the RLS equations 71-73 vary depending on whichvehicle mass estimation equation is used, (e.g., equation 65, 68, or69).

The following parameters are used when the RLS equations (71-73) arecalculated based on the vehicle mass estimation of equation 65:

$\begin{matrix}{{\theta = M_{e}}{\phi = {a_{x}^{s} + a_{x}^{off}}}{y = \frac{T_{pwt\_ whl} - {K_{brk}P_{brk}}}{R_{w} - F_{drag}}}} & (74)\end{matrix}$

The following parameters are used when the RLS equations (71-73) arecalculated based the difference based vehicle mass estimation ofequation 68:θ=M _(e)ϕ=Δa _(x) ^(i)|_(Δt)y=ΔF _(whl)|_(Δt)  (75)

Alternatively, the following parameters are used when the RLS equations(71-73) are calculated based the differential based algorithm ofequation 69:

$\begin{matrix}{{\phi = {{\overset{.}{a}}_{x}^{i} = \left( {a_{x}^{s} + a_{x}^{off}} \right)^{\prime}}}{y = {{\overset{.}{F}}_{whl} = \frac{\left( {T_{{pwt}_{whl}} - {K_{brk}P_{brk}}} \right)^{\prime}}{R_{w}}}}} & (76)\end{matrix}$

The vehicle system uses the RLS to estimate vehicle mass at certainqualified vehicle events. The mass estimator is developed based on thecondition when vehicle dynamic is predominantly longitudinal. Thus thevehicle system searches for conditions under which the vehiclelongitudinal inertia effect is significantly dominating the relatednoise factors, or the signal to noise ratio is high. These qualifiedevents generally occur during vehicle braking/deceleration or duringvehicle acceleration.

In one or more embodiments, the vehicle system detects a qualifiedvehicle braking/deceleration event when each of the following conditionsare satisfied:

1. V_(x)≥C_DynMassBrkEntry;

2. a_(x) ^(i)≤C_aRGEAxNeg;

3. |r|≤C_small_yaw; and

4. (Fwhl<C_FBrk and {dot over (F)}_(whl)<C_DFBrk and {umlaut over(F)}_(whl)<C_D2FBrk) or (Fwhl<C_FBrk2 and {dot over(F)}_(whl)<C_DFBrk2).

In one or more embodiments, the vehicle system detects a qualifiedvehicle acceleration event when each of the following conditions aresatisfied:

1. V_(x)≥C_DynMassPropEntry;

2. a_(x) ^(i)≥C_aRGEAxPos;

3. |r|≤C_small_yaw; and

4. (Fwhl>C_FProp and {dot over (F)}_(whl)<C_DFPropUp and {dot over(F)}_(whl)>C_DFPropDn and {umlaut over (F)}_(whl)<C_D2FPropUp and{umlaut over (F)}_(whl)<C_D2FPropDn) or (Fwhl>C_FProp2 and {dot over(F)}_(whl)>C_DFProp2 and {umlaut over (F)}_(whl)>C_D2FProp2)

In the above conditions, a variable that begins with “C” indicates acalibration parameter. The calibration parameters are predeterminedvalues designed to target a specific estimation event. The conditionsprovide a tunable construction of qualified predominantly longitudinalvehicle motion. The more strict the parameters are tuned towards dynamicsignificance, the higher the estimation accuracy that may be achieved.However, such strict parameters will reduce the available estimationtime interval, since a qualified event will be less frequent. Thus aqualified estimation event depends on a certain level of drivingaggressiveness. The more forceful or rapid the vehicle acceleration orbraking is, the more dominant the inertia effect will be excited in thevehicle dynamic. The tuning of the qualified estimation parameters is atradeoff between estimation accuracy and event occurrence, sinceaggressive driving behavior is less common during a normal drivingprocess.

With reference to FIG. 19, a method for estimating the vehicle mass isillustrated in accordance with one or more embodiments and is generallyreferenced by numeral 1910. The method is implemented using softwarecode contained within the VSC, according to one or more embodiments. Inother embodiments, the method 1910 is implemented in other vehiclecontrollers, or distributed amongst multiple vehicle controllers. Themethod 1910 is applicable to both acceleration and braking/decelerationevents and corresponds to the event conditions listed above.

In operation 1912 the vehicle system evaluates the vehicle speed (Vx) todetermine if Vx is greater than an estimation entry speed threshold.Operation 1912 corresponds to the first event condition. The estimatedentry speed threshold corresponds to a calibrated brake entry speedthreshold (C_DynMassBrkEntry) for the braking event; and to a calibratedpropulsion entry speed threshold (C_DynMassPropEntry) for theacceleration event. If the determination at operation 1912 is positive,the vehicle system proceeds to operation 1914.

At operation 1914 the vehicle system updates the estimation model input(ϕ) and output (Y) based on the vehicle mass estimation algorithmselected, as described with reference to equations 74-76. At operation1916 the vehicle system computes the estimation condition signals ({dotover (F)}_(whl) and {umlaut over (F)}_(whl)), as described withreference to equations 75 and 76.

At operation 1918 the vehicle system evaluates the longitudinalacceleration (a_(x) ^(i)) and the yaw rate (r) to determine if thevehicle dynamic is primarily longitudinal with a sufficiently small yawrate. Operation 1918 corresponds to the second and third eventconditions. For a braking event, the vehicle dynamic is primarilylongitudinal if the longitudinal acceleration (a_(x) ^(i)) is less thanor equal to a calibrated negative acceleration threshold (C_aRGEAxNeg).For an acceleration event, the vehicle dynamic is primarily longitudinalif the longitudinal acceleration (a_(x) ^(i)) is greater than or equalto a calibrated positive acceleration threshold (C_aRGEAxPos). For botha braking event and an acceleration event, the yaw rate (r) issufficiently small if the magnitude of the yaw rate (|r|) is less thanor equal to a calibrated small yaw rate threshold (C_small_yaw). If thedetermination at operation 1918 is positive, the vehicle system proceedsto operation 1920.

At operation 1920 the vehicle system evaluates the force at the wheels(Fwhl) along with higher order derivatives of the force at the wheels({dot over (F)}_(whl) and {umlaut over (F)}_(whl)) to determine if aqualified estimation event is present. Operation 1920 corresponds to thefourth event condition.

For a braking event, the fourth condition is satisfied if: the wheelforce (Fwhl) is less than a calibrated brake force (C_Fbrk); and thederivative of the wheel force ({dot over (F)}_(whl)) is less than aderivative of the calibrated brake force (C_DFBrk); and a secondderivative of the wheel force ({umlaut over (F)}_(whl)) is less than asecond derivative of the calibrated brake force (C_D2FBrk).Alternatively, the fourth braking condition may also be satisfied if thewheel force (Fwhl) is less than a second calibrated brake force(C_Fbrk2); and the derivative of the wheel force ({dot over (F)}_(whl))is less than a derivative of the second calibrated brake force(C_DFBrk2).

For an acceleration event, the fourth condition is satisfied if: thewheel force (Fwhl) is greater than a calibrated propulsion force(C_FProp); and the derivative of the wheel force ({dot over (F)}_(whl))is less than a derivative of the calibrated uphill propulsion force(C_DFPropUp); and the derivative of the wheel force ({dot over(F)}_(whl)) is greater than a derivative of the calibrated downhillpropulsion force (C_DFPropDn); and a second derivative of the wheelforce ({umlaut over (F)}_(whl)) is less than a second derivative of thecalibrated uphill propulsion force (C_D2FPropUp); and the secondderivative of the wheel force ({umlaut over (F)}_(whl)) is less than asecond derivative of the calibrated downhill propulsion force(C_D2FPropDn). Alternatively, the fourth acceleration condition may alsobe satisfied if the wheel force (Fwhl) is greater than a secondcalibrated propulsion force (C_FProp2); and the derivative of the wheelforce ({dot over (F)}_(whl)) is greater than a derivative of the secondcalibrated propulsion force (C_DFProp2); and the second derivative ofthe wheel force ({umlaut over (F)}_(whl)) is greater than a secondderivative of the second calibrated propulsion force (C_D2FProp2). Ifthe determination at operation 1920 is positive, the vehicle systemproceeds to operation 1922.

At operation 1922 the vehicle system executes an RLS vehicle massestimation algorithm (e.g., using equations 71-73). At operation 1924the vehicle system estimates vehicle mass (e.g., using equation 65, 68or 69). After operation 1924, or in response to negative determinationsat operation 1912, 1918 or 1920, the vehicle system proceeds tooperation 1926, and then returns to operation 1912.

Vehicle mass is unlikely to change when the vehicle is moving. Thevehicle system continues to monitor the vehicle state and carries outestimation updates in the presence of a qualified estimation event. Theestimated vehicle mass is averaged throughout all the estimationinstances with the assumption that the vehicle mass is constant and theestimation noise is zero mean Gaussian. However, vehicle mass may changewhen the vehicle is moving slowly and when it is stopped for a certainperiod of time. Under such vehicle conditions, the vehicle systemadjusts the sensitivity of the RLS estimator to a more sensitive state,such that a change in vehicle mass is readily detected.

With reference to FIG. 20, a method for resetting vehicle massestimation parameters is illustrated in accordance with one or moreembodiments and is generally referenced by numeral 2010. The method isimplemented using software code contained within the VSC, according toone or more embodiments. In other embodiments, the method 2010 isimplemented in other vehicle controllers, or distributed amongstmultiple vehicle controllers. The method 2010 resets vehicle massestimation parameters under certain vehicle conditions for adjusting thesensitivity of the RLS estimator.

At operation 2012 the vehicle system evaluates the vehicle speed (Vx) todetermine if the vehicle speed is less than a vehicle stop speedthreshold. If the determination at operation 2012 is positive, thevehicle system proceeds to operation 2014 and increments, or counts upthe vehicle stop/standstill timer (Tmr_stand) by one cycle time and thestop/standstill timer will start recording the vehicle stopped duration.At operation 2016 the vehicle system resets a drive-off timer(Tmr_drive), by setting Tmr_drive equal to zero.

At operation 2018 the vehicle system evaluates a total count ofqualified estimation events (Cntr_EstTot), to determine if Cntr_EstTotis greater than a calibrated estimated event threshold (C_QuaEstEvnt).If the determination at operation 2018 is positive, the vehicle systemproceeds to operation 2020 and updates a RLS estimation memory(Vmass_memo) by setting Vmass_memo equal to a present vehicle massestimate (Vmass_est). Vmass_est corresponds to the vehicle mass(Vmass_rls) estimated at operation 1924 of FIG. 19, and the estimatedvehicle mass (Mv) of equations 65, 68 and 69, according to one or moreembodiments. After operation 2020, or in response to a negativedetermination at operation 2018, the vehicle system proceeds tooperation 2022.

At operation 2022 the vehicle system evaluates the stop/standstill timer(Tmr_stand) to determine if Tmr_stand is greater than a calibratedstandstill timer (C_StandstilTm). If the determination at operation 2022is positive, the vehicle system proceeds to operation 2024 and resetsthe RLS parameter and mass estimation boundaries to their originalvalues. For example, the vehicle system may set the RLS estimationparameter P(k) (from equation 73) to its initial value P(0). Atoperation 2026 the vehicle system resets the total qualified estimationinstance counter (Enter_EstTot) by setting Enter_EstTot equal to zero.If the determination at operation 2012 is negative, the vehicle systemproceeds to operation 2028.

At operation 2028, the vehicle system evaluates the vehicle speed (Vx)to determine if Vx is greater than a drive-off speed threshold. If thedetermination at operation 2028 is positive, the vehicle system proceedsto operation 2030 and increments, or counts up the vehicle drive-offTimer (Tmr_drive) by one cycle time, and the drive-off timer will startrecording the vehicle moving duration. At operation 2032 the vehiclesystem evaluates the drive-off timer (Tmr_drive) and the stop/standstilltimer (Tmr_stand) to determine if both Tmr_drive is greater than acalibrated drive-off timer (C_StandstilTm); and if Tmr_stand is greaterthan zero. If the determination at operation 2032 is positive, thevehicle system proceeds to operation 2034. At operation 2034 the vehiclesystem resets a stop/standstill timer (Tmr_stand), by setting Tmr_standequal to zero. After operations 2026, and 2034, or in response to anegative determination at operations 2022, 2028 or 2032, the vehiclesystem proceeds to operation 2336, and then returns to operation 2012.

A brief or transient qualified estimation event, is insufficient forvehicle mass estimation. Generally, a transient event does not satisfythe second condition related to longitudinal inertia. However, even ifsuch a transient event does satisfy the second condition, the vehiclesystem still removes a transient event from the estimation resultsbecause it will take a certain period of estimation presence before theestimated vehicle mass starts to converge. The estimated vehicle mass isupdated when a sufficient number of estimation instances have beobserved.

With reference to FIG. 21, a method for updating vehicle mass estimationparameters is illustrated in accordance with one or more embodiments andis generally referenced by numeral 2110. The method is implemented usingsoftware code contained within the VSC, according to one or moreembodiments. In other embodiments, the method 2110 is implemented inother vehicle controllers, or distributed amongst multiple vehiclecontrollers. The method 2110 updates vehicle mass estimation state whenthe qualified estimation event exceeds a predetermined time interval.

At operation 2112 the vehicle system determines if a qualifiedestimation event is present. Operation 2112 corresponds to operations1912-1920 of FIG. 19. If the determination at operation 2112 is positivethe vehicle system proceeds to operation 2114, and computes Vmass_rls asdescribed in operation 1924. At operation 2116 the vehicle systemincrements an estimation instance counter (Cntr_est) by one cycle time.At operation 2118 the vehicle system evaluates the estimation instancecounter (Cntr_est) to determine if Cntr_est is greater than or equal toa calibrated time interval threshold (C_EstEvnt4Update), whichcorresponds to a sufficient instance of estimation for an estimationevent. If the determination at operation 2118 is positive, the vehiclesystem proceeds to operation 2120 and updates an estimation update state(Vmass_update) by setting Vmass_update equal to the present vehicle massestimation state (Vmass_rls).

At operation 2122 the vehicle system evaluates the estimation instancecounter (Cntr_est) to determine if Cntr_est is equal to the calibratedtime interval threshold (C EstEvnt4Update). If the determination atoperation 2122 is negative, the vehicle system proceeds to operation2124 and increments the estimation instance counter (Cntr_est) by onecycle time. If the determination at operation 2122 is positive, thevehicle system increments the accumulative counter (Cntr_EstTot) byCntr_est, which counts up at each instance in qualified estimationevents.

At operation 2128 the vehicle system evaluates the accumulated count ofqualified estimation events (Cntr_EstTot) to determine if Cntr_EstTot isgreater than a calibrated qualified estimation event threshold(C_QuaEstEvnt). If the determination at operation 2128 is positive, thevehicle system proceeds to operation 2130 and sets the dynamic massestimation output (Vmass_est) equal to the RLS estimator state(Vmass_rls). If the determination at operation 2128 is negative, thevehicle system proceeds to operation 2132 and sets the dynamic massestimation output (Vmass_est) equal to the stored estimation memoryvalue (Vmass_memo).

If the determination at operation 2112 is negative, a qualifiedestimation event is not present, then the vehicle system proceeds tooperation 2134. At operation 2134 the vehicle system decreases theestimation instance counter (Cntr_est) by one cycle time. At operation2136 the vehicle system evaluates the estimation instance counter(Cntr_est) to determine if Cntr_est is less than or equal to zero. Ifthe determination at operation 2136 is positive, the vehicle systemproceeds to operation 2138 and sets the present vehicle mass estimationstate (Vmass_rls) equal to the estimation update state (Vmass_update).After operations 2130, 2132, 2138, and in response to a negativedetermination at operation 2136, the vehicle system proceeds tooperation 2140, and then returns to operation 2112.

With reference to FIG. 22, the vehicle system limits the vehicle massestimate using dynamic boundaries according to one or more embodiments.The presence of a single inaccurate vehicle mass estimation instance maydistort the estimation state. Therefore the vehicle system establishesboundaries to avoid biased and unreasonable estimation results caused bycertain noise factors in the estimation process. As shown in theillustrated embodiment, the vehicle system sets an upper boundary (Ubd)and a lower boundary (Lbd). According to one or more embodiments, theupper boundary (Ubd) corresponds to a maximum gross vehicle weight(GVW), which includes the vehicle, passengers and cargo; and the lowerboundary (Lbd) corresponds to a value that is less than the vehicle“curb” weight, which only includes the vehicle. As shown in FIG. 22, theUbd corresponds to a GVW of approximately 2,500 Kg, and the Lbdcorresponds to a Curb weight of approximately 1,700 Kg at initialconditions, with a difference of approximately 800 Kg at (time=0 s).

The boundaries are dynamic in that they converge towards the estimatedvehicle mass. Since vehicle mass generally does not change while thevehicle is moving, the boundaries narrow or converge toward a movingaverage of the estimated vehicle mass such that the difference betweenthe two boundaries decreases as more estimation information becomesavailable to filter out unreasonable vehicle mass estimates. Forexample, as shown in the illustrated embodiment, at time 150 s, the Ubdhas decreased to approximately 2,200 Kg, and the Lbd has increased toapproximately 1,750 Kg, with a difference of approximately 450 Kg.

The estimation boundaries will be reset to the original upper and lowerlimits when the reset condition is satisfied as described above withreference to FIG. 20. Additionally, the boundaries will converge whilethe vehicle is accumulating qualified estimation updates. As describedabove with reference to FIG. 21, the vehicle system includes a counter(Cntr_est) that increments every time a qualified acceleration ordeceleration event occurs. A mass estimation event flag (Mevent) isillustrated on FIG. 22, and scaled with the other signals such thatMevent is equal to “NQ” when no qualified event is detected, Mevent isequal to “QA” when a qualified acceleration event is detected (e.g., atpoint 2212); and Mevent is equal to “QB” when a qualified braking eventis detected (e.g., at point 2214). Once a qualified event is detected,the counter starts counting up, once per execution cycle. Once the totalnumber of counts exceeds a predetermined value the vehicle system setsthe initial estimated vehicle mass equal to the RLS estimated mass asreferenced by numeral 2216.

With reference to FIG. 23, as the vehicle system accumulates morevehicle mass information, the estimated mass converges upon an actualvehicle mass. FIG. 23 depicts test results taken for a vehicle having aknown mass, which is represented by the actual mass waveform(Mass_actual). FIG. 23 also includes the vehicle speed (Vx), and thequalified event flag (Mevent) waveform. Mevent is equal to “NQ” when noqualified event is detected, Mevent is equal to “QA” when a qualifiedacceleration event is detected; and Mevent is equal to “QB” when aqualified braking event is detected. The qualified braking eventscorrespond to vehicle deceleration as referenced by numeral 2312. Theinitial estimated vehicle mass (Vmass_est) is referenced by numeral2314. The estimated vehicle mass (Vmass_est) converges towards theactual mass (Mass_actual) as additional information is accumulated, asreferenced by numeral 2316.

FIG. 24 is another graph illustrating vehicle mass estimation. Althoughthe vehicle mass estimate (Vmass_est) converges toward the actual mass(Mass_actual) over time as additional information is detected, thevehicle mass estimate may adjust away from the actual mass when there isnoise present on the input signals, as referenced by numeral 2412. Forexample, in one or more embodiments, vehicle mass is estimated based ona drag force (Fdrag) as described above with reference to equation 65.The accuracy of the drag force estimate decreases under certain vehicleconditions (e.g., high wind). The boundaries described with reference toFIG. 22 reduce the impact of such noisy conditions, by limiting theestimated vehicle mass.

The vehicle system may estimate a dynamic road gradient based on theestimated vehicle mass, for example by using equation 60. Additionallythe estimated vehicle mass may be used for other vehicle systems, suchas brake apply and release detection (BARD) for micro-HEVs havingstart/stop functionality.

As such, the vehicle system provides advantages by estimating thevehicle mass independent of the road gradient estimation using anevent-seeking strategy. The mass is estimated at vehicle conditions, orqualified events, where the signal to noise ratio of certain inputs ishigh. The vehicle system then estimates vehicle mass using a recursiveleast square (RLS) algorithm when the sought event occurs. The vehiclesystem re-initializes certain parameters in the mass estimationalgorithm at vehicle conditions corresponding to potential mass changes(e.g., when the vehicle stops) in order to increase the sensitivity ofthe algorithm. The vehicle system estimates a dynamic road gradient(RGE_(dyn)) based on the estimated vehicle mass. Generally the dynamicRGE has lower quality conditions as compared to static and kinematic RGEbecause it involves more signals in its estimation, and the noise anderror in each signal will be propagated in the final RGE output.However, the dynamic RGE does not rely on the longitudinal accelerationsignal and therefore it has different signal coverage from that of thestatic and the kinematic estimations. Additionally, the vehicle systemestimates the vehicle mass independent of the road gradient estimation,thereby isolating noise in RGE_(dyn) from the vehicle mass estimate.

As described above, with reference to FIG. 4, the vehicle systemdetermines a quality factor for each of the static, kinematic anddynamic RGE algorithms, which are referenced by the variables: QF_(st),QF_(kin), QF_(dyn) respectively. The vehicle system also determines anoverall RGE quality factor (QF_(RGE)). Since each RGE algorithm relieson different input signals, they each have different ranges andavailability.

The estimation quality factor for the RGE output as well as that foreach RGE algorithm (static, kinematic, and dynamic) corresponds to avalue between 0-3, and is represented by variables (QF_(RGE), QF_(st),QF_(kin), and QF_(dyn)). The quality factor evaluates both theavailability and the accuracy of the estimated information. The qualityfactor (QF) factor is selected from a range of 0-3, where a qualityfactor of three (3) indicates full quality where the estimationcondition is satisfied. A quality factor of two (2) indicates degradedquality when the estimation condition is not fully satisfied but theestimation state does not deviate too far from its setpoint. A qualityfactor of one (1) indicates that no qualified estimation has beenachieved near the instantaneous spot and that the estimation state hasnot achieved sufficient convergence, a quality factor of zero indicatesestimation not available.

The vehicle system evaluates the RGE input signal quality andprocessing, which are referenced by numerals 414 and 412 in FIG. 4. Thevehicle longitudinal acceleration is an input signal 414 and is measuredfrom an inertia sensor 52 (shown in FIG. 1). The signal is regarded as agood signal if its quality factor (QF) is equal to three (3) and thesignal is within a specified operating range, e.g. +/−2 g. The inputsignal is locked to its latest high quality value when its qualitydegrades and it will be reset to a default value when the degradedquality persists for longer than a time threshold. Other input signalslike, brake torque, powertrain torque, vehicle speed, yaw rate, etc. areall processed in the same manner.

The static RGE algorithm estimates road gradient based on thelongitudinal acceleration input (a_(x) ^(s)). Static RGE is suited forvehicle standstill conditions, such as engine start/stop functionalitywhen a micro-hybrid is stopped on an incline. The vehicle systemevaluates the static RGE quality, based on the acceleration sensormeasurement signal's stability. The sensor stability is indicated by thesignals oscillation and variation level as compared to thresholds on itsfiltered first order derivative. The highest quality is achieved whenthe vehicle is in standstill and the vehicle body is stabilized suchthat the accelerometer output is near a converged steady state. A higheroscillation level will degrade the estimation quality and becomeunavailable when the preceding kinematic estimation fails to set thestatic estimation's initial value in the current vehicle stop event.

The kinematic RGE algorithm estimates RGE based on input correspondingto the vehicle's kinematic properties including the relationship betweenvehicle speed, accelerations and yaw rate. The algorithm processes theinput using a Kalman Filter. The kinematic RGE algorithm is suited fornormal vehicle motion conditions (e.g., Vx>5 kph) because some of theunderlying calculations have low accuracy at low vehicle speeds.Additionally, a Kalman filter is a dynamic filter, a good estimation isusually not available until the estimation state has converged. Thetransient time depends on the difference between the initial state andthe true road grade state as well as the estimation conditions like:signal frequency component richness and signal to noise ratio. Thevehicle system evaluates the quality of the kinematic RGE based onassessing the quality of its input signals, its estimation state,vehicle state as well as the estimation settle down time period.

The dynamic RGE algorithm 426 estimates RGE based on input correspondingto vehicle speed, wheel torques (brake torque and powertrain outputtorque), and other drag forces acting on the vehicle (e.g., aerodynamicdrag, rolling resistance and the road gradient load). Generally thedynamic RGE has lower quality conditions as compared to static andkinematic RGE because it involves more signals in its estimation, andthe noise and error in each signal will be propagated in the final RGEoutput. Similar to the kinematic algorithm, the quality the dynamic RGEestimation is also evaluated over its input signal, estimation state,vehicle states, estimation qualification conditions, and estimationconvergence/settle-down time period.

The signals that are provided to the Kalman filter based estimator(e.g., RGE_(kin)) are grouped into two sets: the primary signals and thesupportive signals. The primary signals are used to compute the inputsand outputs of the estimator system equation. The supportive signals areused to adjust the estimator mode or parameters to be adaptive todifferent driving situations.

When any of the primary signals are not ok, (e.g., QF<3), the estimationresult will start deviating from the true value. The vehicle systemevaluates the impact of the degraded signal to the estimation state bycounting the time period during which degraded primary signals arepresent, which is referenced by variable T_(dgd). The vehicle systemalso interpolates a degradation impact factor C_(dgi) with respect toT_(dgd). Such an interpolation may be based on predetermined data storedin a look up table. In general, the principle of determining c_(dgi) isthat the longer the presence of the degraded primary signals, the worsethe impact on the estimation results. An estimation quality indexvariable (c_(est) ^(c)) is calculated according to equation 77 as shownbelow:q _(est) ^(c) =T _(dgd) *C _(dgi)  (77)

Alternatively, a pre-calibrated value of q_(est) may be used and theestimation process will be halted in the presence of degraded primarysignals to the estimator.

After the primary signals all resume good quality, the estimation doesnot converge back to the true estimation target state immediately. Theestimation results can be used confidently after a converging phaseoccurs. In order to represent the converging phase, a fading factor,f_(dgi), is designed that takes a value between 0 and 1 to decrease thevalue of q_(est) that has been accumulated from a previous degradedsignal event such that:q _(est) ⁰ =q _(est) ⁰ *f _(dgi)  (78)

This computation is repeated in each control implementation cycle togradually decrease the value of q_(est) down to zero.

At each moment, the occurrence of a present impact increasing can occursimultaneously with the decreasing of the impact from a previous event.Thus, the final estimation quality index is determined by the maximumvalue among q_(est) ⁰ and q_(est) ^(c), i.e. q_(est)=max(q_(est) ⁰,q_(est) ^(c)).

With reference to FIG. 25, a method for estimating the quality of aKalman estimator, or an RLS dynamic mass estimator, is illustrated inaccordance with one or more embodiments and is generally referenced bynumeral 2510.

In operation 2512 the vehicle system evaluates a road gradientestimation (RGE) signal to determine if the estimation is locked. An RGEis locked when it is not updating. The RGE may be locked to its latesthigh quality value when its quality degrades. If the determination atoperation 2512 is positive the vehicle system proceeds to operation 2514and locks the quality factor of the evaluated estimation (QF_est) to itscurrent value. The variable QF_est is a generic value corresponding tothe quality factor for kinematic or dynamic road gradient estimations.When the estimation algorithm does not satisfy a qualified estimationcondition and the estimation is locked, its associated qualityevaluation is locked as well until a next estimation event will happen.If the determination at operation 2512 is negative, the vehicle systemproceeds to operation 2516.

At operation 2516 the vehicle system evaluates the road gradientestimation (RGE) signal to determine if the estimation is active. RGE isactive, if it is updating. If the determination at operation 2516 isnegative, the vehicle system proceeds to operation 2518 and sets thequality factor to a value corresponding to “no data”, (e.g., QF=0). Ifthe determination at operation 2516 is positive, the vehicle systemproceeds to operation 2520.

At operation 2520, the vehicle system compares the evaluated qualityindex (q_(est)), as described above with reference to equations 77 and78, to an OK threshold parameter (Q_est_ok), to determine if q_est isless than Q_est_ok; and there is no supportive signal degradation. Ifthe determination at operation 2520 is positive, the vehicle systemproceeds to operation 2522 and sets the quality factor to a valuecorresponding to “OK”, (e.g., QF=3). If the determination at operation2520 is negative the vehicle system proceeds to operation 2524.

At operation 2524, the vehicle system compares the evaluated qualityindex (q_(est)) to a FAULT threshold parameter (Q_est_degrade), todetermine if q_est is greater than or equal to Q_est_degrade. If thedetermination at operation 2524 is positive, the vehicle system proceedsto operation 2524 and sets the quality factor to a value correspondingto “FAULT”. If the determination at operation 2524 is negative thevehicle system proceeds to operation 2528 and sets the quality factor toa value corresponding to “DEGRADED”, (e.g., QF=2). After operations2514, 2518, 2522, 2526 and 2528, the vehicle system proceeds tooperation 2530, and then returns to operation 2512.

With reference to FIGS. 26A-26C, the vehicle system monitors variationin the kinematic road gradient estimation (RGE_(kin)). FIGS. 26A-26Cinclude 3 graphs of measurement data taken over a common period of timeand are generally referenced by numeral 2610. FIG. 26A illustratesvehicle speed (Vx), FIG. 26B illustrates an actual road gradient(RGE_(ref)) along with an estimated road gradient (RGE_(EST)). TheRGE_(EST) corresponds to an arbitrated output (RGE_(out)) of RGE_(st),RGE_(kin), and RGE_(dyn) according to one or more embodiments. FIG. 26Cillustrates a calculated percent error between RGE_(EST) and RGE_(ref).

The Kalman filter based estimation, such as that used for RGE_(kin),assumes that the road gradient state is stable and is close to aconstant state in a short period of time or distance. This assumption isgenerally true, because most roads do not have rapid changes in roadgradient. However, as shown in FIGS. 26B and 26C, large estimation errormay occur at transitions between different road gradients. For example,the road gradient changes from approximately 0% to approximately 7%, asreferenced by numeral 2612. A generally large estimation error ispresent at this transition, as referenced by numeral 2614.

The vehicle system includes a method, (not shown) for monitoring anestimation variation to determine the estimation quality (accuracy)based on the magnitude of the road gradient variation. In this method, aderivative of the estimated road gradient is calculated (RĠE_(EST)). Atransition period is identified, when the derivative (RĠE_(EST)) isgreater than a threshold estimation and such condition is true forlonger than a predetermined period of time, the transition period isidentified. Subsequently, the estimation quality (QF) will be reduced toa degraded level (QF=2) if no other degradation factor is also presentsimultaneously. Such degradation will be cancelled when the derivativesignal settles down to a low value for another settling time period. Ingeneral, this method informs other vehicle systems that are using theRGE output, that the estimation is not fully accurate in the transitionperiod of time when road gradient changes significantly in a shortperiod of time.

The road gradient estimations are not available at very low vehiclespeeds, as shown in column 6, of Table A. Although none of the estimatesare available at very low vehicle speeds, (e.g., less than 5 mph), thevehicle system still provides an accurate road gradient estimate duringsuch conditions. At low vehicle speeds the vehicle speed provided by orderived from a sensor signal, (e.g., wheel speed sensors or GPS), is notaccurate. The vehicle system stops the estimation at such speeds, toavoid propagating this inaccuracy in the road gradient estimation,(e.g., RGEkin and RGEdyn), and locks the estimator state to its latestqualified estimation result.

A vehicle may travel an extended distance at such very low vehiclespeeds. Therefore the actual road gradient may change after the RGE isstopped. In such conditions, the accuracy of the locked RGE output willdecrease. Therefore the vehicle system downgrades the associatedestimation quality accordingly. The vehicle system defines two vehiclespeed thresholds, (V_(stp) and V_(exit)), which represent a vehicle stopspeed threshold, and a vehicle exit speed threshold, respectively. WhereV_(stp) is greater than V_(exit). When the vehicle speed (Vx) is lessthan V_(stp), The vehicle system (using a Kalman filter (or a leastsquare type of filter) based estimation algorithm) halts or stops theestimation by locking all of the estimation states. However, afterrecording the data at the estimation stop moment, the estimationalgorithm continues, before the vehicle speed (Vx) further decreasesdown to V_(exit), or before Vx increases above V_(stp). When operatingin the speed range between V_(exit)<V_(x)<V_(stp), the new estimationoutput (RGE_(out)) will continue to compare the locked estimationoutput. If the difference between current RGE state and the locked RGEis greater than a first difference threshold, the estimation qualitywill be downgraded to a lower level indicating that the estimationoutput may not be accurate enough in this new situation. However, if thedifference between them is greater than a second difference threshold,the estimation quality will be downgraded to a “No Data” level (e.g.,QF=0) indicating that the estimation result cannot be used. After thevehicle speed (Vx) decreases further below V_(exit), the estimation willbe fully stopped and no more estimation activity will be carried out.Once the vehicle speed (Vs) increases above the V_(stp) threshold, thelocked and recorded estimation states will be reloaded back to theestimation algorithm to resume normal estimation function.

The vehicle system stops the road gradient estimator once the vehiclespeed (Vx) is less than V_(exit), in one or more embodiments, because anestimation at such conditions would not be accurate. V_(exit) may bereferenced as a “crawling” speed. However, the vehicle may continue totravel at such a crawling speed.

With reference to FIGS. 27A-27G, the kinematic or dynamic road gradientestimator will be available after the vehicle speed increases above ahigher speed level and the static RGE estimator can take theresponsibility after the vehicle is fully stopped. FIGS. 27A-27G include7 graphs of measurement data taken over a common period of time and aregenerally referenced by numeral 2710. FIG. 27B illustrates vehicle speed(Vx); FIG. 27E illustrates the static quality factor (QF_(st)); FIG. 27Fillustrates the kinematic quality factor (QF_(kin)); and FIG. 27Gillustrates the quality factor of the arbitrated road gradientestimation (QF_(RGE)). The static RGE is available when the vehicle isstopped. A vehicle speed of zero is referenced by numeral 2712 and acorresponding high quality RGE_(st) is referenced by numeral 2714.Additionally the kinematic RGE is available when the vehicle is movingin normal motion as referenced by numeral 2716 and a corresponding highquality RGE_(kin) is referenced by numeral 2718. The arbitrated RGEcombines RGEst and RGEkin to provide a high quality output over mostvehicle conditions, as referenced by numeral 2720. However, at certainvery low vehicle speeds, as referenced by numeral 2722, there may be nohigh quality RGE available, as generally referenced by numeral 2724.

However, there may be no RGE algorithm available in the low speed rangebetween 0 and V_(exit). Thus, the road gradient variation while thevehicle is moving at such low speed cannot be detected and estimated. Inorder to assure RGE's robustness in reporting road gradient information,the vehicle system includes a conservative countermeasure to avoid aroad gradient estimation being used without providing informationregarding the quality of the RGE.

The vehicle system includes a method, (not shown) for monitoring vehiclespeed (Vx) in a crawling event, according to one or more embodiments. Acrawling event occurs when Vx is less than v_exit (e.g., less than 5kph) and the current estimator quality evaluation is OK (e.g., QF=3).The vehicle system calculates a crawling distance (Dc), by integratingthe vehicle speed. The vehicle system downgrades the quality factor todegraded (QF=2) when Dc is greater than a first distance threshold. Thevehicle system downgrades the quality factor to no data (QF=0) when Dcis greater than a second distance threshold. The vehicle system resetsthe crawling distance (Dc) to zero, when any RGE estimator startsproviding a qualified output. Meanwhile, the quality of RGE will bearbitrated to the current active estimator's quality evaluation resultto replace the quality evaluated in the crawling event. The vehiclesystem will restart the crawling distance computation, when the qualityevaluation (QF) downgrades in the speed range (v_exit<Vx<v_stp).Additionally, the vehicle system restarts the Dc computation when thecontinued estimation result deviates from the locked estimation resultmore than a certain threshold, even if the vehicle speed is greater thanV_(exit).

As such the vehicle system provides advantages over existing system byevaluating the quality of input signals, road gradient estimations andmass estimations and providing quality factors associated with eachinput and estimation. The vehicle system arbitrates or selects anestimation based in part on the quality factors. The vehicle system thenprovides a road gradient estimation and a vehicle mass estimation with acorresponding quality factor based on the selected estimation. Othervehicle systems may use the road gradient and vehicle mass estimationsdifferently, depending on the associated quality factors.

As demonstrated by the embodiments described above, the vehicle, vehiclesystem, and method provide advantages over the prior art by arbitratingor selecting a road gradient estimation from a number of differentestimations (static, kinematic, and dynamic). By arbitrating betweenthese different estimations, the vehicle system provides a road gradientestimation over a wide range of vehicle conditions. The vehicle systemalso provides advantages over existing systems by estimating static roadgradient using dynamically filtering of the longitudinal acceleration,and by using another estimation for the initial value; which reduces thetime delay before a quality static road gradient estimated value may beprovided. The vehicle system also provides advantages over the existingmethods by estimating kinematic road gradient using an EKF, whichprovides an integral type filtering strategy. Such an integratingstrategy does not amplify noise on the signals, and therefore providesimproved estimation accuracy with relatively fast convergence. Thevehicle system also provides advantages over the existing methods byestimating kinematic road gradient using input compensated accelerationoffset to improve accuracy to remove uncertainties and offsets caused bylateral vehicle dynamics and their coupling effects. The vehicle systemprovides advantages by estimating vehicle mass independent of the roadgradient estimation using an event-seeking strategy. The vehicle systemre-initializes certain parameters in the mass estimation algorithm atvehicle conditions corresponding to potential mass changes (e.g., whenthe vehicle stops) in order to increase the sensitivity of thealgorithm. The vehicle system estimates a dynamic road gradient(RGE_(dyn)) based on the estimated vehicle mass. The vehicle system alsoprovides advantages over existing system by evaluating the quality ofinput signals, road gradient estimations and mass estimations andproviding quality factors associated with each input and estimation. Thevehicle system arbitrates or selects an estimation based in part on thequality factors. The vehicle system then provides a road gradientestimation and a vehicle mass estimation with a corresponding qualityfactor based on the selected estimation. Other vehicle systems may usethe road gradient and vehicle mass estimations differently, depending onthe associated quality factors.

While the best mode has been described in detail, those familiar withthe art will recognize various alternative designs and embodimentswithin the scope of the following claims. Additionally, the features ofvarious implementing embodiments may be combined to form furtherembodiments of the invention. While various embodiments may have beendescribed as providing advantages or being preferred over otherembodiments or prior art implementations with respect to one or moredesired characteristics, those of ordinary skill in the art willrecognize that one or more features or characteristics may becompromised to achieve desired system attributes, which depend on thespecific application and implementation. These attributes may include,but are not limited to: cost, strength, durability, life cycle cost,marketability, appearance, packaging, size, serviceability, weight,manufacturability, ease of assembly, etc. The embodiments describedherein that are described as less desirable than other embodiments orprior art implementations with respect to one or more characteristicsare not outside the scope of the disclosure and may be desirable forparticular applications. Additionally, the features of variousimplementing embodiments may be combined to form further embodiments ofthe invention.

What is claimed is:
 1. A vehicle comprising: a controller configured togenerate output indicative of a road gradient derived from a firstgradient estimation when a vehicle speed is less than a speed thresholdand a signal indicative of longitudinal acceleration sensor inputquality exceeds a threshold, a second gradient estimation when thevehicle speed exceeds the speed threshold and the signal exceeds thethreshold, and a third gradient estimation when the signal does notexceed the threshold.
 2. The vehicle of claim 1 wherein the roadgradient is independent of a vehicle pitch angle.
 3. The vehicle ofclaim 1 further comprising: an inertial sensor for providing an outputindicative of the longitudinal acceleration; wherein the first gradientestimation comprises a static road gradient estimation based on thelongitudinal acceleration.
 4. The vehicle of claim 1 wherein the secondgradient estimation comprises a kinematic road gradient estimation basedon the vehicle speed and a longitudinal acceleration.
 5. The vehicle ofclaim 1 wherein the third gradient estimation comprises a dynamic roadgradient estimation based on the vehicle speed and at least one of awheel torque and a wheel force.
 6. The vehicle of claim 1 wherein thecontroller is further configured to: generate the output indicative ofthe road gradient corresponding to an estimation having an estimationoutput that has updated within a predetermined period of time.
 7. Thevehicle of claim 1 wherein each estimation further comprises anestimation quality factor, wherein the estimation quality factor isindicative of an availability and accuracy of a correspondingestimation; wherein the controller is further configured to select theestimation based in part on the estimation quality factor.
 8. Thevehicle of claim 7 wherein the controller is further configured to:select the first gradient estimation when a first estimation qualityfactor corresponding to the first gradient estimation is equal to avalue indicative of full quality.
 9. The vehicle of claim 7 wherein thecontroller is further configured to: select the second gradientestimation when a second estimation quality factor corresponding to thesecond gradient estimation is equal to a value indicative of fullquality.
 10. The vehicle of claim 7 wherein the controller is furtherconfigured to: generate an output indicative of a road gradient qualityfactor based on the estimation quality factor of a selected estimation.11. A vehicle system comprising: a controller configured to: generateoutput indicative of a road gradient based on a selected estimation thatis a static estimation when a vehicle speed is less than a speedthreshold and a signal indicative of longitudinal acceleration sensorinput quality exceeds a threshold, a kinematic estimation when thevehicle speed is greater than the speed threshold and the signal exceedsthe threshold, and a dynamic estimation when the signal does not exceedthe threshold, and generate output indicative of a quality factorcorresponding to the selected estimation.
 12. The vehicle of claim 11wherein each estimation further comprises an estimation quality factor,wherein the estimation quality factor is indicative of an availabilityand accuracy of a road gradient value resulting from a correspondingestimation; wherein the controller is further configured to select theroad gradient value of the estimation having an estimation qualityfactor that is greater than the estimation quality factors associatedwith the other estimations.
 13. The vehicle system of claim 11 wherein astatic quality factor is based on a derivative of the longitudinalacceleration.
 14. The vehicle system of claim 11 wherein a kinematicquality factor is based on quality factors associated with the vehiclespeed and the longitudinal acceleration.
 15. The vehicle system of claim11 wherein a dynamic quality factor is based on quality factorsassociated with the vehicle speed and at least one of a wheel torque anda wheel speed.
 16. A method for arbitrating road gradient estimation,comprising: generating, by a controller, output indicative of a roadgradient based on a static estimation when a vehicle speed is less thana speed threshold and a signal indicative of longitudinal accelerationsensor input quality exceeds a threshold, a kinematic estimation whenthe vehicle speed is greater than the speed threshold and the signalexceeds the threshold, and a dynamic estimation when a longitudinalacceleration signal does not exceed the threshold, and generating, bythe controller, output indicative of a quality factor corresponding to aselected estimation.
 17. The method of claim 16 further comprising:reducing an estimation quality factor associated with at least one ofthe kinematic estimation and the dynamic estimation when a derivative ofthe corresponding estimation value is greater than an estimationthreshold for longer than a predetermined period of time.
 18. The methodof claim 16 further comprising: locking the road gradient to a presentvalue of the selected estimation in response to the vehicle speed beingless than a low speed threshold.
 19. The method of claim 18 furthercomprising: reducing the quality factor when a difference between alocked road gradient and the selected estimation is greater than adifference threshold.
 20. The method of claim 19 further comprising:setting the quality factor to a low value indicative of the roadgradient not being available, when the difference between the lockedroad gradient and the selected estimation is greater than a seconddifference threshold, wherein the second difference threshold is greaterthan the difference threshold.